I am using a TSP custom cam and the I/E lift is .635". However, they do offer lobes that have lift of .655/.657 and I am wondering how much whp , if any, that I am giving up with the slightly lower lift on my current cam.

I am using AI ported heads. Here is some information that I found on their flow numbers.

I believe your duration and LSA will have more of a determining factor. Lots of LS7's out there with lifts < .650" who make plenty of power. If you're gains are so high in the RPM band that you rarely ever see anyways, you're not actually "realizing" those gains.

My cam is pretty mild: 236/250 LSA 115+4

I am sure that I give up some peak whp with the +4 advance but I am not that familiar with how lift factor in whp numbers.

If I had this same same cam with a .655 lift, would I see any difference in mid range or peak whp and if so, approximately how much.

.02" more lift with the same duration and lobes would not be a big difference. That is not even as much lift difference as going from a 1.7 to 1.8 rocker on an early LS (LS7 is already 1.8).

I'd guess 5hp tops, if that? Definitely under 10hp.

If I were using stock rockers I would just stay at .635" lift, engine longer valvetrain life, give up a couple hp