You sure the et30's are the same width ? just had a look around and it appears that et30 ones are 9j and et15 are 9.5j, that's where you'll lose the...
I very much doubt there will be any difference on the concave between et30 and et15, dish and concave is usually done on wheel width, offset is done...
I had a 231 for a short while and thought it sucked big time as a drift car, really hard to keep it drifting as it just bogged down all the time and...