I got a little over 41 fpe with JSB 33.95 grain pellets with mine out of the box. 740fps. I later found a burr in the barrel port and when I removed it I got another 40 fps on 20 grain FTTs. I did not reshoot the 33.95s. I detuned mine to shoot the FTTs at about 875 fps. It is right at 32 fpe now. But if you want more fpe, I would test the 31 grain H&N Baracudas and the 33.95 JSBs (and possibly variants). My guess is you might get 800 fps on the 33.95s with a small regulator increase and/or small plenum increase. In other words, low to mid 40 fpe out of the box, high 40s to possibly low 50s with a little work. I wouldn't try to go higher, I think it would be a lot of work for not much change. These days I plan to shoot a 10 shoot string with new pellets recording the velocity of each shot and put them into two 5 shot groups. Unless the ES is well under 10 I will probably reject the pellet. Even if the groups are not great, however, if the ES is low I'd shoot it some more. I've found my groups are a lot less consistent than my velocity measurements. The pellets which shoot low ES have ALWAYS given me the best accuracy. I like to shoot 30 yard challenge targets and I think they are a better way to test accuracy than groups. At least for me.
This thread started out with a regulator complaint. With their favorite pellets, all three of my P35s will shoot 10 shot strings with a ES around 6 or 7 and 30 shot or more strings with an ES of about 15. With pellets they don't like, the ES is much bigger. Like maybe 40 fps. 2 to 3 times larger. I don't think you can judge the regulator very well unless you have tuned the hammer spring to give you peak velocity or slightly less and you are shooting a pellet your gun shoots well. I don't know why regulators react this way to the guns tuning but at least on my P35s they definitely do.