no experience with upgrades or with SI driving. Based on experience from the middle SII setup I can only add a few data points and remarks: As for the data points let’s start with braking power. Brakes can’t be any better than the tires on the tarmac. If you can produce a 4 wheel blocking at any speed the brakes need no upgrade. I did two wheel blockings braking down from 70 mph and felt like there were reserves. Never tried 100+ mph though.
Fading: Early SI tests found fading - maybe that’s why Jaguar put on vented discs on SIII. I haven’t done much alpine downhill driving so far and never provoked fading issues
Vibrations: Early SI tests found vibrations when braking down from high speeds (100+ mph) and attributed them to the soft connection of suspension to body. Maybe harder bushings can set this straight.
Braking effort: The SII power assistance is comfortable, but linear to a high extent. For city driving the tip of your big toe is sufficient. Yet, if you need more braking power you can step in harder and receive effective braking as described. Very modern cars, in particular compact cars, tend to over-assist braking on the first touch. You’re impressed, until you really need braking power and find out that there is hardly any more provided, no matter how hard you brake. - Of course, this is all related to the fact that most rear-ends are caused by little determined drivers stepping on the brakes too little and too late. Anyway, getting back to the SII brakes, they are clearly easier to handle than the unassisted brakes in the Spitfire and pretty close to the 2012 family hauler. And that’s all it takes for me.
Maybe you compare the parts nos for the boosters (my part no. is RTC1298 for RHD, relates to RTC1127 for LHD) - I wouldn’t invest into a SIII upgrade, unless maybe you run across a complete 1992 V12 ABS setup …
75 XJ6L 4.2 auto (UK spec)