It doesn't cap "too soon" as much as "semi-auto-caps then doesn't have anywhere to go". GK does fairly consistent WS dmg regardless of what you fight, so it's going to be at it's relative strongest vs higher level mobs and relatively weaker vs trash mobs. Hagun gets ~12%+ more melee dmg, tomoe gets ~the same in increased WS freq after you take into account meditate and such (this actually works out in tomoe's favor given WS:melee distribution).
atk: 624 lvC: .4 def: 335
base fSTR WSC sum fTP cratio pDIF avg dmg
hagun 75 16 92 183 1.975 1.900 1.880 815.375
pole 86 8 30 124 5.100 1.230 1.076 816.419
So you want to be in the 620+ atk range on average and capped acc to even consider switching to polearm vs merit lv monsters (w/o piercing weakness). For weaker HNM you're going to want to average 680+, and for something sky god level you'd want to be in the upper 900s (and still not hurting for acc, which I doubt is possible). It's also important to note that once you push atk high enough w/ DRK+Chaos/berserk, you'll be capped on the high end (high roll + berserk) but not on the low points (berserk cool down + low roll) making your true avg lower than what it's approximated to be. Obviously this is going to shift a bit for gear/merits/skill/race/whatnot as well.