Gristle... This is cRPG.
Your idea is bombarded by one, unless it buffs one's playstyle. That's how it seems to be, at least.
The idea is great, I would SURELY loom/buy loomed boots for the sake of +2 athletics.
15 athletics reachable by going pure agi and loomed boots... YE-HA! That's what we need to buff agi chars.
Oh, and to not make it overpowered, every boots at masterwork give you +2 athletics, but we need some kind of restriction so people don't loom black-armor boots and run around with 10 athletics (if they build correctly).
Weight-based restrictions? Or maybe all boots would be simply split into two groups:
- Light boots - those earn athletics as an additional stat through heirlooming
- Heavy boots - those include middle-weight boots, too and earn some other additional stat through heirlooming instead of athletics