I've never heard of them being worthwhile. I had the same idea but those amps are designed to work with the infinity speakers only and if you use it to power any other kind of aftermarket speakers, they will just sound like crap IMO. I've tried replacing speakers in those systems and they tend to blow them up and sound awful. Just use the stereo as you have it (minus the amp) and it will sound great with a new set of speakers.
Also, a great resource for car stereo wiring diagrams is www.the12volt.com
You can find anything you ever wanted to know about car stereos on there, subs, amps, wiring diagrams, etc.