i used 185/70/14 but still it used to lock up, never tried bigger wheels, but i am sure 195/65/15 wud be better, also the suspension and struct shud be stock, no extra weight (cng) in boot..
people have tried replacing front brake calipers with that of axio/prius/camry along with brake distribution T valvle and say there is some improvement.
some have replaced rear disk brakes with drums also claimed improvement,
i never try to play with factory fitted brakes, too risky and complicated for me