If you start changing baffle dimensions and layout, then you potentially have to redesign the crossover network to accomodate the different actual response. The tweeters will respond differently on a different baffle than designed by Danny.
That's the perfectionist way of thinking.
The truth is, there are variances in drivers from kit to kit, and those will likely be greater than the .5db or so diffraction ripples. Unless all you care about is a ruler flat response curve, you could probably offset the tweeters without it doing any harm, whether it does any good or not you wouldn't know without measuring.
My opinion, it'll look funny with an offset tweeter in an MTM with the flush mount mids designed to overlap the tweeter. What would do more good for sure rather than offsetting the tweeter would be to make a double thick front baffle and put a bigger roundover, like 1" or more on it. You could also put a felt ring around the tweeter if you're worried about diffraction - but then again, the shape of the T2 tweeter faceplate does a lot to negate the effects of baffle edge diffractions.
About the ribbon, it's not a drop in. It'll fit a 104.5mm cutout, that's where the drop-in ends
It will take a totally new tweeter crossover network, and the shape of the cutout is different behind the flange. They do sound nice, but honestly the T2 is a pretty nice sounding tweeter as well.
If you want to improve on an A/V3 kit, I'd say the best way to do it would be with robust cabinet construction. If you have all the measurement gear and know-how, you could tweak the crossover to your taste, and to accomodate variances in the drivers, but that's a whole new level
