There is an upgrade, don't know if it's still offered that can substantially increase the power of a Rex p, search for African air ordnance. It makes the gun bigger though because it uses a larger rear tank.
What I don't like about them is the underlever style cocking. It exposes the trigger and makes engaging the safety after each shot a must. The mechanical parts of this mechanism are exposed, sand and dirt could easily get in and make it crunchy. The only thing holding up the lever when closed is a ball bearing in a recess, there's lots of slop on it that I don't care for. Also the stock handle I found to be very uncomfortable and crappy although they can be swapped for most AK style grips i think.
There's some nice features like a burst disk on the tank. It's a pretty simple but solid design not too unlike an airforce air gun. I haven't checked those in a while but if the airforce talon p comes in larger calibers now id probably recommend one of them over the Rex p.
The xp pistols may only come in super large calibers, might have to ask them is a smaller caliber could be made, I bet it could