I had a similar issue, which I didn’t pick up on right away because I rarely wind my XJR-100 up over 2,500 RPM. I put gas in the car and got the message that the performance was… Something… I can’t remember the exact verbage but it meant the car is gonna run like S!@t unless I fixed the problem or aka limp-home-mode.
Quick visual inspection that requires minimal amounts of toolage.
Look at your exhaust tips. if the tips are black and dripping sooty yucky stuff, then it’s running rich enough to cause the computer to puke a code. stick your finger in the goo (if there is any) and smell it. if it smells like unburned fuel, it’s a tad rich, if it smells a bit sweet along with that unburned fuel note (smell), could have coolant entering the combustion chamber - don’t panic it doesn’t necessarily mean a head gasket.
Check the air box for obstruction, check the condition of the air filter, if you’re using a K&N (which I wouldn’t recommend) then pick up some MAS cleaner and give the airflow sensor a gentle squirt of cleaner (it’s important that you use MAS cleaner or an electronics cleaner because these cleaners won’t leave a residue which could “damage” the sensor. Brakleen? That would be a No! Also don’t touch the Mass Airflow Sensor with your hands / fingers or any other body parts. The oil in our skin is enough to skew the MAS reading into a lean P0171 P0174 (OBD Code) or both. That’s why I don’t recommend running a K&N or any other oil soaked reusable filter. I learned this the hard way. 
Check the condition of the ducting to the supercharger, make sure there aren’t any cracks or breaks in the ducting. This can be visually done. For the less obvious, a squirt bottle with water in it. or a can of carb cleaner (I use water because I don’t want to run the risk of the fluid in the supercharger igniting). With the car running, squirt the ducting to the Supercharger and any other gasket or hardware air related mated surfaces that would or could permit air to be sucked in on the wrong side of the MAS. If there’s a leak, the idle will change.
Bust out the toolbox
Pull a coil pack or two and inspect the spark plug, a lean condition will be a chalky white with a slight brown tint. Rich condition… well… will be black, and the electrode may have build up of carbon on it. A perfect burn will be a light taupe(y) brown color. All of the above is based on the fact the plugs aren’t new.
Question
Do you have a OBD tool? If you do, plug it in and review the codes. I have the service / diagnostic manual in PDF format where you can do a code look up. I can send you if you want. Depending on the scan tool, it will also provide the reading from the O2 sensor as to the air-fuel mixture.
If you don’t have a Scan Tool - get one, any Automotive Supply house will have them - not so cheap. I picked up a cheap one at Harbor Freight for $45.00, and yes its crap, but it does what it needs to do.
Anyway, back to me (My wife is always telling me "this isn’t about you, I respond Yes it is! :P)
The scan tool reported back the P0172 and P0175 codes which equates to both banks running too rich (BTW, these P0172 and p0175 are OBD standard codes - and yes my HF scan tool did recognize the Jag codes. Didn’t know what to do with the codes other than list the code(s) along with ‘MSC’, and no MSC doesn’t mean miscellaneous, like the tool holding a the scan tool initially thought. It means Manufacturer Specific Code.
I checked the MAS, the O2 sensor, and the throttle positioning sensors all checked out in spec. Hmmm…
I Picked up a can of Royal Purple fuel system cleaner (This stuff is awesome BTW). The problem didn’t immediately go away, it took about a 1/4 tank of gas to make the code go away, but the performance still sucked. then the codes came back. P0172 P0175. Hmmm… About the same time I’d received a coolant low light. What The ____! Ok, fine.
Back under the hood.
Upon inspection I could see the leak was around the Coolant reservoir. Off with all of the plastic pretty cover up bits, Sure enough, the pretty plastic bit had severed the small return coolant return line back to the holding tank. Hmmm… The timing (not ignition timing) was poor cause I needed to be someplace. and my beloved Xj6 S-III had been put out of commission by a woman in her hubby’s monster truck. 
Anyway, I found a piece of Fuel hose and a couple small hose clamps in the garage, and repaired the leak, until I can order and replace the tiny plastic hose. While I was moving stuff around to ensure that wouldn’t happen again, I noticed the wiring harness for the coil packs and injectors had also fallen victim of the evil plastic cover. The cover had rubbed through the electrical taped (wiring harness). As I was prepping the harness for a repair, I also discovered that it had also rubbed through the insulation on some of the wires in the bundle - yes these are the wires for the coil packs and fuel injectors. In the immortal words of Adam Savage (Mythbusters), “Well There’s Your Problem!” I really didn’t want to purchase a new wiring harness.
I cut the frayed wires and soldered bits of wire of the same size where the insulation was rubbed off, covered with heat shrink and re wrapped. Done!
all of that being said, I would also recommend checking the wiring (harness) to the coil packs and injectors to make sure they are in good nick.
So there you have it. My two cents. 
Mark