I've used both the Intense and DHP PCM'* in my car, and my milage didn't really get affected by either, even though they are quite different from each other.
You can get an Intense unit for 100 bucks. DHP is over 200, but they are pretty much geared toward W-bodies (GP, Impala, Monte, Regal).
As for your warranty, it could void it, but for the most part, the dealers don't really know if you would have an aftermarket PCM unless they had to work on it, in which case, they would reflash it if there was a problem and you would lose the aftermarket code.
It'* your call if you want to do this or not.