The only way you can play games which require OFW 5.55 and up, is with the GEN decrypters. If you really want to play this game, you should update do
5.50GEN-A, then GEN-A build 4 (same link as before), then
GEN-B, then
GEN-B2.
Then you can use the decrypter to play your game.
By the way, if you're worried about all you plugins, don't be, 5.50 GEN has compatibility with most, if not all, of the most popular plugins for CFW, including some of the more obscure ones.
Note: the updater for GEN-B and GEN-B2 look exactly the same, so do them one at a time, i.e. copy GEN-B onto your memory stick, and run it, then delete it, then copy GEN-B2 and run it. Also, when you update to GEN-B2, the system info doesn't actually say GEN-B2, but if you can run gPSP then you've got GEN-B2