I haven't done the LP mod, so I can't comment on much personal info with that. Basically, you turn your HP reg into a high volume cell. You'll need another regulator (in the foregrip most likely) if you do the LP mod.
Without the LP mod, I shot about 4000 rounds with a 68/3000 tank (output pressure of 500psi) and had to refill my tank about 3.5 times. That comes out to roughly 1150 shots per tank fill.
I haven't used my Timmy in the rain. I'm afraid to.
It was VERY damp and humid when I played - no problems. I hear it has to really pour for it to be a problem. I haven't heard any reports of light drizzle knocking the guns out.
I love this gun.