It is noticeable. I did the same switch as you with the RS2's. The 235/40's have visibly less sidewall than the 225/45's. It's not a lot, but I can definitely tell the difference. It was worth another "finger" of wheelwell gap- the 225/45's are a half inch taller. That being said, I much prefer my 235/40's. The difference in response and gearing is noticeable. They're wearing quite well, too... I was not aware of the compound difference. Interesting.
The 235/40's also have a minimum recommended wheel width of 8", but they work great on my 17x7.5 rotas. No squish.