I also have a Gemtech M4-02 on an 11.5" BCM I've been using about 5-6 years. I don't notice gas in my face with the M4-02 or the SDN-6. Admittedly I"m pretty insensitive to it. Have you tried the RTV trick on your charging handle to decrease the gas leakage? The sound on it is pretty comparable to the M4-02 or M4-2000. I bought this one for my MWS primarily, but also so I could run it on a few of the 5.56 guns. I haven't gotten a chance to run it on the MWS yet. I've got finals coming up so it's going to have to wait until they are done. Then I plan on spending a day out at K&M putting it through the paces on both the MWS and a couple of other guns.
The worst gun I have the issue with my Sig 556 SBR, I've thought about trying to do some RTV'ing on it around where the upper meets the lower in the rear but never have gotten around to it. After a few rounds my eyes are watering pretty bad with the Sig. It isn't nearly as bad with my longer rifles, but my 10.5" AR definitely needs some RTV attention too.