The holster you are looking at is more for patrolling. It has the air strap and then two other straps to hold the gun in place. Even if you only use one strap, it is still going to be a slow draw, thus the patrolling use.
If you want something fast, look at a safariland or the Blackhawk kydex on a platform. Something that has a thumbreak or the Blackhawk finger lock thingy or just passive locking. People who use drop legs for competition use those normally, because they can draw them just as quickly as they would if they were on their belt.
That holster is very popular along with the spec ops one which standard issue. But both of those are for patrolling, not speed.
As far as comfort, keep it tight and you shouldn't have a problem. Never put a drop leg mag holder on, so I dunno. I'd get a some type of a chest rig, It'd be alot quicker as you wouldn't fumble trying to grab a mag. Something simple like the one one of the posters put a picture of.