If you're shooting in the standard arms extended position, the few inch difference in sight length doesn't make any difference. Also, the further that the sight is mounted forward, the more barrel heavy the gun gets, and the harder it is on the wrists for extended shooting sessions. I used to shoot NRA bulleye pistol with a bull barrel Ruger MKII, and I wouldn't have wanted the extra weight any further forward than directly over the grip.
I can't speak specifically for the P17 since it's a handgun, or for air guns as a rule but..I mounted my CCO on my M4 as far forward as possible. It was setup similar to a scout scope. I found target acquistion and accuracy much improved as opposed to the CCO close to my eyes. Try it and see is what I recommend. The only reason I haven't tested it on an air rifle is I don't use dot sights with them.