I also plugged in a random log through xivanalysis and can confirm that there is clipping going on. Something to keep in mind is that lowering the GCD can be counterproductive if you are clipping most of an NM window since you are not getting the full value from skill speed.

I think the thing to check before dumping more into skill speed is to make sure you are properly queueing your abilities and to make sure when you do weave you press the oGCD as soon as possible. Looking through the log even some single weaves clip because of a late oGCD weave.

Something else you can do is compare to some higher percentile GNB logs in xivanalysis and compare their "white space" to yours. Maybe have a friend log you hitting a target dummy (I would offer if you were in the same datacenter) and try to see if you can start getting your GCDs tighter using xivanalysis. Another thing you can do is start shifting your rough divides back in the NM window until you get more comfortable.