Not sure the Mithril Golem NEEDS the boost, but I probably will. Though not all of them - obviously he wouldn't get the various Body promotions (the Golem equivalent to weapons) since he'd already be considered to have Mithril Body (being as he's MADE of the stuff) And field promotions are probably OP - Golem Engineers would probably be a late-tech unit, possibly even a National Unit. The Workshops would come along earlier, but being an improvement would be restricted to your own borders (which are by definition closer to your cities and thus less drastic an improvement)
I'll admit it, my working theory is this - if every force has some sort of overpowered option, nobody's truly OP because each of them has their own counter-option. Also, I'll probably slow their tech capabilities a bit by spreading out their unit tiers - they'll still be using Metal Golems when others are using later-game tech, but on the other hand their Golems will be able to promote much faster and be much more customizable to match up against even stronger forces. Their strongest point is the time in-between, where they have the same tier as everyone else (though some promotions will be unlocked through tech - they'll have points that will more or less be "Tier x.5" in that they now have promotions to bridge the gap) and at the very end of the tree, where they have both strong units and the promotions to back them up. An end-game army with Archmages, Golems, and Golem Engineers should be a dangerous thing - just like Auric Ascended, or any of the other big late-game options.