it requires greater weapon focus as a prereq, which itself requires 8 fighter, so it is impossible to take it with 5 fighter
Ah OK. But it's technically not what OP had written.
100% of Enhancements (as per OP) would mean they would have the Capstone, which is Level 20 fighter, so that wouldn't really apply in their case unless if they skipped the feat requirement (which I agree is a weird one to place in the middle of a chain upgrade). Also, it's not a T5 that has this issue directly that OP's wording suggested, it's an indirect issue most certainly, but details matter here for someone looking things, otherwise they'll look and won't see an issue because they can only check in relation to what you write. (Again, details matter)
But I can see what you (and from that what OP) are on about and agree that it was a strange design choice. Again, especially given it's the middle of the upgrade path in the tree.
Can a Dev take a look at possible alternatives to this Feat requirement? Such as the Weapon Master T5 enhancement only granting +2 to Damage and Attack, but if you have the necessary Feat you gain another +2 to Damage and Attack and then the +25% HP bonus? That way players still getting to Fighter Level 8 (12 Total) will achieve the requirements as now, but would allow anyone else who skimps on Level 8 Fighter and the required Feat to still gain the Enhancement and some bonus from it.
J1NG