I dunno if the outcome would be quite so one-sided. Remember that forge worlds require massive influxes of new materials to keep the factories running. It would all depend on how well the adeptus mechanicus could defend their warehouses and assembly lines, and if they get a good defensive line up early (remember that the Titans are at least as big as the Commander, and probably equivalently armed), the Commander would be forced to scrounge scraps and may find itself on the wrong foot.
Oh, titans (barring maybe the scouts ones, I guess) are probably
much larger than the commanders and almost doubtlessly better armed -- the D-Gun has a hell of a kick to it, but other than that all they've got is a single laser. Commander's big thing is the construction/command capability, and TA tech base would be able to do things with materials on hand that I'm not sure WH40k's tech could even
dream of (and remember, given enough time a commander can just direct the construction of a massive metal maker/fusion farm -- they can basically go completely reclaimless if they really feel like it). A forge world would probably be the equivalent to a metal world in TA, and those just get
silly.
Core would also have the fun potentially of possibly setting up Krogaths for the go, and, well. Those probably are as large as the titans and similarly armed
You'd have to do a lot of futzing around insofar as weapon powers and suchlike goes, though. I don't recall how much in the way of hard(ish) numbers TA had...
Though I'd probably call it ultimately for the Imperium, likely via exterminatus. It'd depend on a lot of factors, really... Commander managing to actually assimilate Imperium tech could cause gods know what. The TA construction capability is basically goddamn magic and them setting up to spew out Imperium warships and Titans (enhanced however with TA tech, of course -- ARM getting a hold of a geneseed, ferex, might cause
hilarity) could cause some craziness. There's just... a lot of variables involved, heh.
'Course, if a diplomatic solution of some sort
was reached... game over.
Any of the WH40k sides getting a hold of TA construction technology would be a game ender for everyone else. Barring maybe the 'nids, who'd probably just eat it. 'Course, ARM cloning getting a hold of 'nid organisms...