I can't give you an "exact" comparison, but I run 2 RL-P 15's ported and GF's car has 2 assassin 8's ported.
Obviously mine hits much lower and harder, but hers is very respectable for a couple of budget 8's. I'd do 4 of them ported as that should give you the SQL you want. Throwing 400+ watts to each sub with NO problems. SQ is very good on them and they will rattle the car when playing bass heavy songs.
3 Assassin 8s will pleasantly suprise you, I guarantee. While I haven't heard the rl-ps I do have 4 Assassin 8s ported @ 34hz and they are nearly as loud as my single Ava 15 on the same power.