what year spx? I'm guessing it is a late 90s with the x4 hull and an 800 motor. Those are nice nimble boats but are completely different handling machine than the RXX which is a much larger hull. The 800 motor is a very solid motor and there a tons of mods for it, still raced frequently. But even with major mods, it probably will just keep up with an RXX or a stock RXP and it will beat you up in rough water more.
I suggest you ride an RXP and see what you think(I doubt you'll find somebody that will let you ride their RXX, evertime I did, it came back messed up). It will cost more money but you can get 04s pretty reasonable now, $4-$5k. An RXX may cost a little less, but not much and probably won't hold its value as well.
I think 05GTX was refering to the RXPX that came out in 08, most people don't know what an RXX is. I think there's one on ebay right now.