5mm will make stuff all difference in this scenario and as said can all be taken up by sag.
Mindmap, Theres a few different things you can do with SXs which improve over the stock setup. The main thing to watch is for the links binding up either on each other or the shock body. I think you could run an 8.5x2.5 and maintain travel with a slightly slacker head angle which would be a good thing...up to 2.5 wide the tyre should clear. I have mocked it up in the workshop and it seems fine but never ridden this setup (also tried the same thing with 8.75x2.75 instead of 9x2.75 in my '08 SX-Trail and that did work well, transformed the handling, much more mini-DH, although I only did it with a crusty old manitou shock for proof of concept).
Cheers for that. Mines an 05 and to be honest I quite like my DHX Air, it works pretty well since being serviced. I'd quite like to try it with a coil shock, but the only one I could find in the stupid size is a Cane Creek DB, which are crazy money.
My only concern with running a shorter shock would be that the BB would be too low. I'm running it in its lowest setting as it is and get a few rock strikes. I guess I could always put it in the steeper mode to try and compensate. Hmmmm.
2015 Stanton Ti Switchback
2006 Specialized Demo 8
2013 Specialized Allez Sport
1999 Santa Cruz Chameleon
1994 Cannnondale Beast of the East (rebuild still in progress)