I want to build a 10.5" upper but I've been reading about the dwell time difference of an extra inch. Supposedly the 11.5" is significantly better at being reliable with less than perfect conditions the 10.5 may choke on.

Would a piston improve the 10.5's potential shortcomings? ETA: looking at the osprey 416 kit.

Dwell time is not going to change because the bullet speed is not affected by the piston vs DI debate.  Most short stroke pistons work on an impulse sent down the piston rod to the carrier.  So really what you're looking at is the amount of impulse imparted on the piston by the gasses while the bullet is dwelling between the gas port and the crown.  There isn't much you can do to change that, so you have to fiddle with the mechanisms to take as much advantage of what gasses you do have.  (Assuming you aren't planning to open the port up more).  Which means trying buffer weights and carrier masses and even buffer spring strengths.

I guess my question back would be, which shortcomings are you trying to improve?

ETA: there are only a few things you can do to change dwell time.  Move the gas port along the barrel, change the bullet speed, and change the barrel length.  To some extent there are frictions at play and the twist rate also, but those are minor changes to the first three.

Whatever variences/shortcomings the 10.5 would be more sensitive to, ie: ammo varieties.

I figured since the ospreys chamber is closer to the port there would be a slight advantge over DI (its chamber being in the reciever). So openeing up the gas port a bit would help? I plan to run suppressed as well, which is why I'm considering the piston kit, I dont like the gas in my face.

Well, a couple things come to mind.  Keep in mind, I've never tried this exact thing out so I'm only speaking from the mechanics of the systems etc.  The Osprey isn't an adjustable system so that's one thing to be clear on.  When you add a suppressor you're going to increase the back pressure of the gasses in the barrel.  In essence, adding a suppressor will increase your dwell time sort of, but not like lengthening the barrel.  What it will do is cause some back pressure toward the chamber which should have the affect of boosting the gasses as they try to exit the barrel, preferably through the gas port.  Which should make your rifle more likely to cycle lower powered rounds, but on the flip side may actually cause standard or higher pressure rounds to over cycle.  Many piston systems have an adjustable gas system to, effectively, reduce the gas port size so that the combination of the increased back pressure with the reduce port size results in a normal cycle rate when using a suppressor.

I would think that you could get it to work right, but you'll have to change the buffer weights and spring strengths to find the sweet spot.  Problem is, you may find the suppressed sweet spot isn't the same as the unsuppressed sweet spot.  This is just a guess but you might find that you have to use a heavier carrier/buffer combination to slow the bolt carrier down so it will cycle well, but then when you remove the suppressor it will be too heavy to reliably cycle lower powered ammo.  That OR get an adjustable gas block.  But too many variables is bound to cause problems.  Pick one or two things to fiddle with.  Don't try to change every single piece because you might end up fighting against yourself.

Here you go, talk to this guy:

http://www.ar15.com/forums/t_3_126/651778_Latest_SBR_Build.html

Not an Osprey, but it's a piston on a 10.5".  My Adams Arms Piston is a 16", so I can't really explain how the shorter barrel will function.