So I called Tippmann today about this same issue because it was driving me crazy. Tippmann told me that the best they achieved on HPA was in the 250 fps range. They said that the TPX was initially designed to run off 12 grams, and the liquid inside.
I dont understand the liquid being particularly something that the TPX needs to function properly but more of a input pressure thing. This is weird and I can only assume that it is because Co2 by nature puts out about 900 psi while a normal high pressure air tank puts out about 850 psi.
So, my theory is that if I raise the pressure in my HPA tank I will be able to achieve higher fps out of my TPX. Sadly, I dont have any shims for my Ninja tank to test with. This might be the only explanation as to why some people have achieved higher playing velocity on HPA.
Just like mags, the operating pressure is just under 400 psi but likes and needs an input pressure higher than the industry standard 850 psi to satisfy its thirst.
This is maybe something to think about if you are planning to run your TPX on remote with HPA. I think an extra shim might do the trick or just run bulk Co2.