Hi 12's or lows 12's?
If you want low to mid 12's youd be up for at least 5 -10 grand on mods i think maybe more. My opinion, get rid of the RB20 and out in an RB30/25 hybrid. I think it would work out cheaper than working the tits off the RB20, your decision. There is a thread on how to do this (R33 RB30 Conversion), i am currently sourcing parts for mine now.