I would try cleaning/relubing the reg, using a spent 12ie instead of a dummy, and starting your FPS testing over with the reg adjusted all the way to zero pressure. Lock down the relief valve as mentioned, then try setting the velocity. Frankly, volume in front of the valve (spent 12g vs. dummy) shouldn't make a difference because the TPX valve has an internal shutoff mechanism, but that's what I was running when I used mine with a remote and I didn't have an issue getting 275 out of mine.
What size paint are you shooting? What type of tank/reg are you using? And lastly, what type of remote line?