With the SHP you can have the 500mm barrel pretty easily shooting 25.4's or Hades or Atomics right near 900 fps. And get around 30+/- shots at that tune (reg right ~100). And the 600mm gun (also with SHP) will do that same thing without even breaking a sweat, and be able to give more. I don't know the reg pressure on my 600/.25 as I prefer to keep my SHP 'clean' of the angled gauge so I can drop the stock when I want. But I do know that the reg was dropped by 1 full turn from where I got it from Talon Tunes, who had tuned and shipped it to me with the standard, small plenum. So after I installed the SHP, I could drop the reg and get more shots before pressure fell off the reg.
For me, 900+/- fps with hollow point pellets is really all I need, and I can see where if woods walking I would prefer the 500 to the 600. And if I read the OP correctly, you are possibly concerned with maneuverability with the small vs the SHP plenums - I just don't see that as an issue. Keeping barrel length down seems way more important to me - and knowing that you will need a pretty big moderator on an unshrouded gun like the DRS, you likely will want to keep your OEM barrel length to a minimum.
Just my 2 cents.