generally speaking, if you have an R12 system converted to R134a, then the performance will be worse CFC's were not used for no reason

not only because the condensing capacity needs to be higher with R134a, but also because the expansion device will not be optimised

some systems will cope better than others, i converted my subaru a couple of months ago and it works pretty well. but then japanese cars tend to have better a/c anyway as it is more important in the domestic market