I agree with Crazy Twizy when it comes to the car wheels. We need to understand that they add weight... really much so. But another aspect is also the wheel angles. We can not really be sure that the Twizy can handle wide and low profile tyres/wheels in a good way without further modification on the suspension. Ofcourse, if you choose to use (as an exempel) 17" wheels with 205/40 tyres and you think/experience that the grip is increased and that you can live with the stiffer ride, then why not continue to do so (without being bothered by other peoples comments). BUT, if you want to optimize the vehicles handling in relation to the comfort, i'm quite sure that 205/40-17" (or something else with wider/lower profile) is not really the best combination. So: 205/40-17" wheighs approx 20kg each. (80kg in total!) Standard twizy wheels is less than 10kg each (less than 40kg in total). This added weight is on a bad position since it is "suspension weight" - meaning that it has to be handeled by the suspension. It also causes flywheel effect meaning slower acceleration in not only one aspects, but TWO (weight + flywheel effect). The only positive effect is that the wheels might give the Twizy a good look and that the wheels have a low center of gravity.
About batteries: i agree about the fact that the Twizy batteries would need to be replaced to something bigger or more powerful if you need it to be REALLY fast, (I'm thinking around 100hp) but the standard batteries could at least handle 30-40hp. The problem is only if you go to a "go-cart circuit" or similar - meaning driving really active under a longer period. With Kenneths Power Box, it is not an issue, because the electric motor will be overheated first, thus saving the batteries. But let's say that we drill some holes in the enclosed motor and add a fan, then there is a chance that batteries might end up in heating issues instead. I say might because I think that the powerbox "only" gives the Twizy approx 18-25hp instead of the standard 13hp so I think it is anyway quite safe ( 13hp was measured on a dyno in a Swedish magasine...so unfortunatley the standard 17hp does not seem to be true...). If 30-40hp is used: i'm not really familiar with the Twizy cell chemistry, and it's corresponding heat resistance, but I would say that if possible, the standard batteries should also be cooled by a fan to prevent fast battery degradation. In the end, I think that a 10-12kWh battery together with a battery cooling fan is needed if we are speaking about 60-130hp. If 40-60hp I would say that just a 10-12kWh battery without a fan would be enough.