Perhaps their could be a benefit for a race car where the small gain outweighs the cost of maintenance. But not so much for road cars.
I think there have been some small Alfas and Citroens with inboard brakes. But if it was truly a performance benefit for tiny cars then Caterham would have fitted them. The last one I built had outboard brakes.
Iāve been reading your replies with interest, it appears that the care and feeding of inboard disk brakes is a pain. But, is that because of access to them with in the confines of the XJS, or an inherant issue with the brakes themselves? I guess what I am getting at, is not to fault the brakes if itās an access issue.
Pat
it was probably a 60s thing carried over from the Etype, and Jaguar couldnāt afford to redesign the rear suspension with outboard brakes. Just like they couldnāt afford to redesign the rear main seal.
Ford came in and did it.
In standard, unventilated, form the inboard rear brakes are not great for braking. Good for cooking the diff seals though. And access is a pain because of the rear sub-frame cage, which you will need to drop if you have to change the disks. Most owners suffer them, I donāt think anyone likes them.
I have brought this up in the past, many years ago. I see that it was a question of sprung to unsprung weight. In other words a suspension question where the unsprung weight is design wise a better way to go for best suspension design. Kirby agreed on that argument but of course there are other considerations and over time and usage, suspension performance was sacrificed for use and application but when suspension performance was a main consideration, the inboard is the best choice and still is when looking at a strictly racing choices.
Itās primarily due to the design of the XJ-S. The saloons had an access panel under the back seat making it far easier to work on the handbrake calipers, but the XJ-S didnāt have that. Aston Martin used the same calipers but located them behind the diff rather than in front, making them loads easier to access.
Thatās probably a fair statement. I certainly wouldnāt seek out a car with IB brakes. Iāve never heard anyone say āI love my inboard brakesā nor āI wish all my other cars had inboard brakesā 
How much suffering is though, would depend on individual experiences, obviously. Iāve never felt like Iāve suffered them; Iāve had no problems with them. To me theyāre just an oddity, worthy of neither particular praise nor particular condemnation.
Cheers
DD
The idea is to reduce unsprung weight and torsional stresses on the suspension. Those factors would mean little to anyone who isnāt using the vehicle for competition. However, those using the vehicle for competition eventually opined that they prefer outboard brakes because sometimes they need to change pads during a pit stop. Itās easy with outboard brakes, just pop out the old and in the new while the wheel is off. With inboards, someone has to get under the car.
Access. If they were easier to repair complaints would be hugely reduced. Itās a pain. If you have to feel the pain every couple years it would be intolerable to anyone. If you have to feel the pain every ten years, maybe not so bad. We all have difference pain thresholds
Cheers
DD
If you are a mechanic who has worked on inboard systems āmanyā times, it is difficult but not impossible to access and change pads. One can also do things to eliminate exhaust heat. The real question is āwere you counting on an easy fix or are you ready to remedy a problem with new fabrications or eliminate a problem with no consideration other than time and money involved?ā. All designs are done with specific trade-offs. Just choose that which best fits your considerations and live with the trade-offs involved. There is really no right or wrong about this.
The brake calipers on the Salisbury diff are bolted to the output shaft flanges. I wonder if itād be possible to go the Aston Martin route of relocating the calipers to the back side of the diff by simply rotating those flanges around, or perhaps swapping them left and right. Youād need to fiddle with brake line routing and handbrake cable routing, obviously, but it might be an interesting option short of swapping in the later outboard brake system.
An excellent alternate idea. Thanks for that. Itāll be interesting to see if anyone goes for this fix.
Based on what youāre saying, DoubleD, if I took Superblack in to my local Just Brakes for their $140 rear pads and rotors special ā¦ what outcome? 
Iām wondering, given the 18-year history of the XJS inboard rear brakes, why didnāt Jag realize at some point non-ventilated rotors were causing bad things to happen back there due to the heat created (e.g. differential seals) and make the very simple switch to those rotors with ventilated design? Or, do they really not spare that much heat? 
They also stuck with those oddball sandwich rotors with a loose iron ring around the outer edge, yielding an odd jingling when the car was moving slowly. Not even sure what the idea was there.
Ring is to reduce squeal.
How many members here have replaced those rotors with conventional solid rotors with no ring and have had any issues with squeal?
But when the customer just paid $40000 for the car new, and the brakes squealed once, they were back at the dealerā¦
Mine still has the OE rotors with the ring, 98000 miles. Pads replaced at least once prior to my ownership. No squeal.
Neon (remember those?) owners complained of brake squeal when I worked at Chrysler dealers. There were many service bulletins over the years addressing squeal on various models. Big warranty expense for the manufacturer. At one point we were changing caliper and pads to Akebono on Jeep Grand Cherokees.
Dodge Neon, the last car my brother owned ā¦ 
It was a good little car, until the timing BELT snapped, and then ka-boom went the engine. 
One thing we can say about our Jags, as much as the issues they may have, is Jag never cheaped out and subbed out the good 'ole reliable timing chain(s) for belt(s). I hate those things ā¦ 
Never had a squeal problem with non-ringed rotors.
Never heard any jingling from the OEM type rotors either.
Iāve just been blessed with noise free brakes, I guess. It comes from good, clean living
Cheers
DD