I don't like your idea of the "modular" bag, Rob. It sounds like it wouldn't be as strong. Shooters Connection bags have inner pockets that you can remove.
I bought one of the 5.11 Tactical "Shooters Connection knockoff" bags. It's a good bag, but it's not as good quality as the Shooters Connection bag. Some of the metal parts don't look particularly solid. I already know of one guy who wrecked the D-ring simply by picking up his bag wrong. Put a lot of gear in it, and it does become heavy.
I looked at the Shootersgear bags. If the material wasn't quite so soft (I like a stiffer bag...get your minds out of the gutter, you perverts!) I'd have picked one up at the CanNats last year right away.