500hp is no problem for the R154 provided it's doesn't have any pre-exiting conditions. It's the next best thing to a V160 as for as manual trannies go.
a v160 in an mk3 is a bad idea u loose gearing as for the r154 they are beast only real issue they have is syncros wearing out and those are fixable 500hp shouldnt even phase it unless it just wore slam out or has a problem already