Gonna get reamed for this suggestion, I'm sure, but here goes. If you really want heirlooms to be special, it's not the first tier of them that does any good, it's the higher tier ones. The ones that gen 11's can get every week or so. How about inverting the gen bonus? First Gen is the fastest to get to retirement, and it slows down a bit every gen after it. This would have the effect of seeing a few more tempered or heavies than now, but masterworks and balanced, etc would become rarer and more special.
So a gen 1 would be able to get an heirloom quicker, but in exchange for the advantage of it, you take a bit longer to get to retirement with bonus, and even more powerful gear.