It will definitely help. Even the competition systems in the low power ranges run such low impedances that the effective output is very high.

You could run the subs, and the rear speakers only off the amp and get another for the front. Or, run all 4 speakers from each channel of that amp and get a new one for the sub itself. Or, my favorite - get a 5 or 6 channel amp that does it all under one shell.

Historically, rear speakers need about half the power of front speakers, and a sub about double the power of the front speakers. It's a good ratio to begin with anyway.

I think not only the increased power, but also getting the sub onto it's own gig would help as well. A 5 channel amp should already be set up that way though... I particularly like the Alpine MRV-F450. I've convinced mafisto to get one for his car.