The reason why smaller wheels with larger profile tyres ie stock sizes ensure optimum handling is for a number of reasons;
1: the cars suspension and handling traits were designed around a given wheel tyre combination.
2: MR2s have MacPherson Struts.
These have terrible camber control under compression which means that the camber varies quite widely when you load a wheel under cornering.
Lower profile tyres give less than a stock profile ie 35 profiles say on an 18 will flex less than a 50 series profile.
The higher profile helps the tyre maintain a good footrpint on the road when the camber varies when the wheel is under load.
3: Lower profile tyres have problems maintaining a good footprint of rubber on the road
(as mentioned in 2) and lower profile tyres have a stifer sidewall
(they have to) so this means that should you exceed the limit of grip that they will break away quicker ie less progressively and you will get less feedback as to when this will happen.
4: Bigger wheels mean heavier wheels which increases unsprung weight.
However, MacPherson struts have a massive amount of unsprung weight anyway which exemplifies the already mentioned camber control problems, so if you hit a bump mid-corner the car is more likely to
'skip' than say a car with a proper double wishbone setup because there is so much unsprung weight the suspension ie dampers/springs will not react as quickly.
5: With bigger wheels and likely wider tyres there will be more grip, so you can go higher speeds into a corner and all the time you are working the tyre below maximum grip levels you will to a degree be able to go faster, however the aforementioned problem of less progressiveness of lower profile tyres combined with increased sprung weight of bigger wheels and of course not forgetting the propensity for MacPherson struts to cause the camber to go all over the shop means that at the point the car starts to handle
(ie just over the maximum limit of grip) everything happens more quickly which makes the car harder to catch.
This is most noticeable in the wet only because it is difficult to exceed the grip levels in the dry.
Of course what makes it worse in the wet is because of the comparitive lack of grip it ends up feeling like the their is no progressiveness at all in the way in which the tyre lets go, so it makes it difficult to predict.
Those that say the car feels like it corners as if on rails are simply talking about roadholding, not handling as they are driving well within the limits of the grip available.
thats basically it.
