What's bad about putting big wheels on my charade?

The first reason that going so big is wrong is basically because tyres/wheels have to work with the suspension:

As the suspension moves through it's travel, various things happen to the susp. geometry: The main problem that going so low in profile gives is that the tyre no longer has sufficient sidewall flexibility to take care of the camber change that occurs during normal suspension operation. This means that as the camber changes, the tyre ends up running on it's edge rather than the tread of the tyre being flat to the road. This can (and does) mean that you end up with LESS rubber on the road during hard cornering where you need it most.

The classic example of this is watching a lowered Escort Cosworth with rubber bands launch off the line: The rear suspension of the EsCos is the same as that on the donor Sierra, namely semi trailing arms. These have a particularly bad camber change during bump travel. This means that the tyre is completely on it's inner edge. This is made worse by the fact that during launch the weight transfer make the rear squat more causing further camber change. You can see the skid mark gradually become thicker from 2" out to the full tyre width as the launch progresses. These muppets actually believe that their traction problems derive from the monster boost they're running rather than their choice of tyre/wheel. The really laughable thing is that they (esp RSOC Muppets) think they know more than you when you developed the goddamn thing in the first place!!! Hehe!

The above is not true of race cars as they 1) don't have McPherson strut/semi trailing arm/other cheap nasty road suspension compromise and 2) have incredibly stiff springs and barely roll during cornering.

Finally, and yes, there is more: to allow suspension to work properly we want the lowest unsprung weight possible. This is not achieved by hanging 17" of styling off each corner and then expecting good traction on a bumpy road. The lightest (and cheapest) wheel tyre combination for a G100 Charade is 185/60 cut slick on a stk steel rim. Stylish? Poss not. Effective. Certainly and you'll drive circles round your mates on their rubber bands. I take it you didn't see me lapping Silverstone last year on this setup?