As far as I know, the Fast Chip slightly alters the fuel curve, bumps the timing (which can be done for free), removes the speed limiter, and may alter the shift points to be slightly higher, although I'm not sure about it.
On the 307's, having the ECM bump up the timing vs the distributor helps as the timing is so high in the midrange that it can't tolerate much more on the initial, but needs a lot more on the high end.
I think the 4.9L is the same way. Needs more at WOT, but bumping up the initial isn't the best answer, allthough is DOES overall help power, BUT, if the ECM would allow for more WOT timing, and lean out top end some, I feel there is more power to be gained.
For the LT1's, the Impala SS community is pretty convinced he does a great job. But the $350 price tag for the LT1's, owch! I'll buy the cables for my OBDII Suburban and my OBD1 FWB for $120 and TunerCat for $50-$60 and be a much happier camper!