Re: my friend just got an rg550, but....
I agree, my 520QS's do sound better, in fact the one I had with stock pickups was my favorite guitar for awhile, but I took it in to be worked on because I was getting scratchy noises with the pickup selector switch, and the switch got replaced, and it's never sounded quite as good since then. I've been thinking of replacing the pickups in my rg570 and rg550 (that I just got). What's stopping me is I already have other guitars that sound the way I like 'em to sound, so the fact that the 550 and the 570 sound the way they do means they're different than my 520qs, et al, and so I'm not sure I really want to try and make all my guitars sound identical. Some variety is good.
But a tonezone in the bridge would probalby be my first try, if I were to swap pickups, based on my tastes. I'm guessing, since you like the 520QS, that you find the bridge pickup a little dull and muddy, and missing that cutting high-end. That's my experience with the S2. Evos are a little too bright and nasally sounding for my tastes, too far in the other direction, though that can be tamed somewhat, and it's a good sound to have in the arsenal, it's just not my fave.
Having said that, I don't think I've actually heard a tonezone in the 550, and I'm not really all that sure what you'd like, so it's hard to recommend something. Choosing pickups is always a bit of a crapshoot. I put a tonezone in a Jackson RR1, and it didn't really do what I was hoping it would do, for example.