Depends what you mean, XJR-S are all post May 88 cars and are more common, the limited versions of the pre May 88 TWR cars had a fuller range of customer ordered performance options and proper ones are pretty rare, there are likely only a tiny handful of fully specced ones with all the inherent problems of trying to maintain serious custom made items.
Most of the things TWR did to the XJR-S cars (aside from the 89 onwards 6.0L engine) to improve performance can be replicated on a standard XJS on a more limited budget but without the inherent problems of running the Zytec engine management system.
All XJR-S cars have modifications to the gearbox to move the shift points higher up the rev range and firm up the shift. However that modification can be done to a standard XJS V12 gearbox via various kits you can get from the US. Changing the shift points is a reasonably simple yet long winded trial and error task through changing the governor springs and inner weights and fitting a variable modulator. It will liven up the car so you can hold gears better into the power band whether at partial throttle or full throttle without kickdown.
Firming up the shifts fully is a bit more involved and requires removing the sump pan and modifying the valve body, so the car needs to be raised at least to do this and the transmission fluid drained.
The 6.0 XJR-S versions have larger air intakes which can be replicated on a standard 5.3 XJS to give a little more power, as can cold air ducting and low loss filters. Intermediate exhaust boxes can be removed to add a bit more power with straight through pipes. The bore of the rear section of the exhaust can be enlarged a bit more and still fit under the axle and better back boxes can be fitted. All in all you can get up to 30BHP extra depending if you want to start modifying the advance as well. That was the basis for the pre May 88 TWR engine efficiency kit.
XJR-S and pre May 88 TWR shocks are bilstein one off custom made so not easily replaceable, there are plenty of "modern" adjustable shocks that will do as good a job with lowering/firming the car. Likewise there are more easily available alternatives to the speedline alloys for fitting bigger tires. TWR bodykits are reasonably readily available second hand to improve the aerodynamics if you like the look. The only real bit that is difficult to replicate is the quicker steering rack with reduced power assist. You can fit modern bushing that will help with the feel but not much more unless you can pinch the rack from a scrapped XJR-S which is a straight swap.
So other than the extra torque and slightly different power curve you get from the 6.0L XJR-S engine and the steering, you can replicate/improve the rest. Unless you seriously want to go down the insanity of trying to find one of the proper TWR hand built engines and custom manual gearbox. Depends if you motivated by owning some form of "genuine" TWR car or just looking for performance enhancements. If its the latter I would just improve a standard XJS and save yourself a lot of hassle.
One other thing to bear in mind is that in mid 87 there were a number of "mini-facelift" styling changes to the V12, like the wood centre console replacing the black vinyl one slight change in switch placement on the shifter, metal door treads replacing plastic ones etc. Also the changes on the HE from Salisbury to Dana back to Salisbury diff in the 80s. Depends whether you consider such things important for which model year you go for.
The V12 car in standard form is not sporty, its a stylish GT designed for very comfortable long distance driving. Yes if you try you can make the car loose the back end in the wet or make some noise if you absolutely floor it and engage kickdown. But if you want sporty the standard auto box will drive you mad, it will upshift into top gear as soon as it can to improve fuel consumption, likewise the diff/gearing is made for efficient motorway cruising not for quick 0-60 times off the line. The handling/cornering is "wallowy" and the steering light IMHO, yet overall the drive is very easy/smooth and the engine with standard exhaust very very quiet (breathing and power is sacrificed for quiet refinement).
I've never driven the manual straight six version so I can't offer a comparison. Anecdotally I have been told that the reduction in weight of not having the V12 with slushy autobox and having better gearing is just about offset by the reduced torque/power so aside from top speed the performance is much of a muchness.
