They're both Q5s, and both textured reflectors. Ran both on new batteries. And I compared the the two on Max/Turbo, as the OP question was about brightness. (I own a P4 P1D, you can tell by eye that it's not the Q5.)
I was going to attribute the P2s slight gain to a deeper reflector, as the P2 has a longer head assembly than the P1, but comparing them side-by-side, they both appear to have the exact same reflector (the P2D Rebel has a deeper reflector). So my guess would be that there's a 5-10% brightness difference that isn't perceptible outside of a long-distance throw test.