That isn't a bad system, but I would still allocate more to the sub. Incidentally, I have the same speakers in my home theater, p362's, c350, and p162 surrounds, but my subs are different (two Hsu VTF3's). I also used to own a STF2, that was a great little sub, but for a large room, I would try to upgrade to the VTF2 at the very least, as I think the STF2 would definitely get lost in a room like that. One thing you could do is go with p162 fronts and p152 rears, take that money saved and upgrade the subwoofer. Trust me, in a swap like that you are gaining a lot more than you are losing.
The subwoofer has to pressurize the whole room, and if you are sitting in the center of the room with sub that can't do that, you will hardly hear it at all, even if it is playing at full blast. That is just the nature of the room acoustics of bass. You might also look into the advantages of near-field placement with subwoofers, ie the sub being used very close to the listening position, like an endtable. This is a way to get a nice bass sound with a smaller sub in spite of a large room. Even in that role, I would still want to get a VTF2 or something with about that level of performance. Good bass performance is the difference between a home theater and a kick-ass home theater.