I believe Colin's are 22mm. The ones from Steve are 24 or 25mm. I have the 24mm one. Remember this isn't a linear relationship either. 25mm is dramatically stiffer than 22mm.
Although it is THE upgrade to make the thicker bar will compromise ride quality on poor roads. Mostly this won't be an issue but on a road which is rutted due to HGV traffic near me, you can feel it. There ain't no such thing as a free lunch, every change has a knock on effect. The first thing you'll notice with your new cornering dynamics is that tall tyres don't half move about in corners! It would be interesting to drive a car back to back with each one fitted. It could be if there aren't any other mods made that there is not much point going with the much thicker bar, just more compromise. Remember also the "correct" way to engineer this is redo the mounting so the bar is linear as before. The thicker you go the more pronounced are the issues related to the transition (taper).
What I also recommended is that you fit new heavy duty springs to the rear. These aren't really very stiff anyway and seem to balance the handling somewhat. It certainly doesn't feel as understeery as before anyway. Plus you car actually put some shopping in the boot without the car squatting like a dog with worms.
I think the roll bar, HD springs and 6" wide wheels (tyres sits better in a wider rim) with 70 profile tyres are about as much as the car will usefully take without serious re-engineering. Whatever we do, the geometry of the front is compromised, which is (one reason) why P7 and beyond were looking to radically change this.
It's interesting to reflect that the cancelled 2000S was supposed to have a thicker roll bar (anyone know the spec (I'm guessing only 1/16"-1/8" or something like that) but remember that a properly engineered car is done so as a complete chassis. What Rover were waiting for was the tyre technology, it would seem in the context of the day, they didn't consider the trade-off worth it. That tech did not arrive on the mass market in the UK until the 70 profile tyres on the XJ6, so really when the S2 was signed off any beyond the point of any worthwhile chassis changes (the car was supposed to go only to 74). Remember even the E-type sat on what were the equivalent today of 185R15s with a full profile.....
