I use PB Natural Look ( straight ) in a small HVLP gun, and it works fine.
Really nice for doing wheel wells and tires.
I just saw a review on the Opti gun today, forget who it was at the moment.
They were shooting PB Bold N Bright ( straight ) with the Opti gun fine. Had to have been the liquid, I would guess the BnB gel would need to be cut if that is what you were using.
Not too sure what the specs are to drive the Opti Gun, it looked nice with the spray pattern he was getting. He had a ~ 20 gal Napa single stage - oil compressor ( my opinion from the shot in the video, he did not recall at the moment ).
The other option some use for car shows is the PB trim restorer.
This takes a bit to dry and is very high gloss. I don't use it for tires, just really bad trim.