I have a G21C
2 flames come out the top of the slide, I dont notice them while shooting it but they look pretty bad ass on video. It shoots slighly flatter then when I run an aftermarket unported barrel in my 21C, but being a .45ACP large framed pistol IMO it tends to be more of a push back rather then a muzzle rise of one of my 9mm Glocks without a comp. I do not have any 9mm factory compensated Glcoks, but I do have a few aftermarket comps I run on my 9mm Glocks and I find those do more as far as recoil mitigation, but I think that is more do to the snappier nature of the 9mm round and smaller frames of my 17 and 19's vs large frame and .45 ACP round of the 21.
Only down side is the inside of the slide gets dirtier since the ports are inside the slide so more of the gasses get inside the slide
A plus over my aftermarket comps is that you do not have to remove the comp to totally dissasemble the slide, the factory comped barrel comes out of the slide like a normal barrel. And if you choose too you can always run a regular unported barrel in it
When I purchased it the comped model wasnt something I was specifcally looking for, but it was on sale for a good price so I jumped on it, and I do not regret purchaing it. I shoot it both with the OEM comped barrel and with an aftermarket uncomped barrel also
