I think both. You will get more air flow and be more consistency.
There is a video on the website showing them shoot a M1100. I donno if it is the modded one or not, but it fires very well.
Pros are: More shots, more power, can fire more bb's at once, more consistent.
Cons are: Too much input pressure and it might break something. You have a line running from your gun to a regulator and airtank on your webbing or somewhere's. That may be a con if you try to hand your gun to someone under fire.
I think Madmax did something of this nature at one point.
link