If it ain't broke, don't fix it. I've never tried the GFS P90s, but I do have an Epi '56 LP. If you have the same P90s (and I would guess you do) my gut feeling would be that you're not going to improve on them on the cheap. After playing with the height a bit, I was totally satisfied with the stock pickups.
"Tragedy is when I cut my finger. Comedy is when you walk into an open sewer and die." --Mel Brooks