I've read a couple of threads relating to "420a vs 4g63" discussions, and a few people have said that at less psi we have more horsepower because of our high compression pistons. So if I get new lower compression pistons and I stay at 5-8 psi will I lose some hp? (I know I can just turn up the boost since I'll be built then but am just curious)

Correct. 8psi on 9.6:1 compression makes more horsepower than 8psi on 8.6:1 compression. It's not a BIG difference in power, but there is a difference. You'll probably never notice it anyway because like you said, with lower compression you can just crank up the boost! :)

I always figured it to be like running one less psi. So on stock comp, if you run 8 psi, it will feel like 7 psi on lower compression. It wont be very noticible at all really, esp if you had a worn motor before the rebuild.