SDJ, I would suggest getting along to your nearest dealer to give it a try. I'm guessing the guys in Cwmbran would be closest and they have a nice blue demonstrator.
Alternatively, further down the M4 you have RTCC who have one you can try.
My gut feeling is it is probably about the same internal dimensions as the SS but I can't say for sure. What I would say about the S5 is that the doors open much better than the S4 for ease of getting in and out especially if you are either of larger build or have back/mobility issues (I always have 2 and sometimes all 3 of those!)
You might want to check to see if there is a difference between the auto and the manual as well because there may be a difference in gearbox tunnel width.
