I've been playing Pathfinder E6 for quite a while. I modified Rise of the Runelords for E6 and completed the entire AP. Did the same with Serpent's Skull and we're currently starting book 6. Personally, I wouldn't want to go with E8 because I think it gets a little out of hand even stopping at 6th. The final encounter for RotRL was CR 14 and was an amazing battle with a party of 4 (all survived). But we were starting to see the negatives of high level play - longer turns, hints of rocket tag, lots of book keeping. Those 20 extra feats with plenty of capstone abilities really added up - my best estimate is roughly +1 APL per 5 additional feats.
A problem I've hit is the undead in the CR 6-9 range who start dishing out negative levels. I opted to change that to Con damage since the PCs can't fix negative levels. Otherwise, E6 has kept things fast paced and the balance seems pretty good.
I don't use exp, so can't help on that. However, you don't want the feats coming too fast because it is very disruptive. I had them come too fast in RotRL - the party got a feat every 2 or 3 encounters near the end which didn't work well.
Next up will be Mythic E6 Legacy of Fire, going to tier 5 or 6.