Like the others said, do it. A Teres 135 is so much better than a P3 your head will swim. If it's still too much $ consider an armless Nott' Horizon for about $1K. Easy to mount a Rega on that too.
You can also upgrade your RB300 with parts from OL, Expressimo, etc. This will take it well above the stock model. I'd suggest a VTA collar first (Expressimo's is best IMO), then a rewire, then an upgraded end stub, then a new counterweight. You can do each of these steps as funds become available. Final performance compared to an OL? I'd ask Twl. :)
If you order a Teres 135, tell Chris Brady you want a 25mm arm hole. If you just say "Rega" he'll drill 24mm and you'll have to ream it out for the VTA collar.