I would be careful with a high pressure tank. I pre-ordered the sniper pump and spoke to an empire rep that mentioned hp is really hard on the seals and reg, which means it'll technically work but cause more wear and require more maintenance. I just use my ninja slp from my g6r for this sniper. My roommate has a sniper too and runs an lp tank at 230. I run mine at 190 and we both chrono the same and consistently at (+-5) 285.
Also, you don't have to buy a whole new tank. You can find cheap lp regs around and swap it with your current tank - or invest in a 50/4500 ninja
btw, besides being a bit loud and a little stiff of a pump action this is a truly awesome pump!
your 68ci/4500psi tank makes no difference in what pressure your reg outputs.