I have both Q5 and P4 in my minimag.
the P4 is easier to install because the metal tabs can touch the inner lip of the minimag, just like the stock setup. The Q5 I had trouble with due to the rectangular base, but eventually I had to mount the Q5 on top of the OEM black plastic assembly.
Also, you need to consider heatsinking. A single 14500 li-ion without any regulation will give each LED ~1.0A. They will heat up quickly and that dummy cell isn't going to help. For my install, I used 1/2" aluminum rod as a heatsink. It's long enough that it acts as a spacer for the battery. The only drawback is that the alum. rod isn't big enough to be touching the mag's body so heat cannot transfer outside..only to the heatsink. It's okay because I seldom turn on my lights for more than 2 minutes anyhow. If you're not going to have a large heatsink, you may want to consider another way of powering the LEDs. I have no regulation on mine and the P4 pulls about 1.0-1.1A while the Q5 pulls around 0.9A on a single 14500 Li-ion. Both are equally bright but I prefer the P4s.
I'm using a 20mm Khatod SMO relfector for the P4. I bought them from Photonfanatic in the marketplace for $4 USD a piece. From what I gathered,the khatod SMO seems to be more favorable when you're going with 20mm or smaller. But for reflectors larger than that, OP is a better choice. I had to lightly shave the reflector in order to fit into the mag. it's nothing tedious, but its the best reflector I have found for the P4s in a minimag.
In addition, I bought a few Terralux tailswitch. Utilizing the OEM method was a hassle and defininitely inconvenient.
