[Autumn]

My biggest issue with the gigantys is that despite being a fuckhuge sword with tons of incentive to make your hits count

Its scaling is worse than a base longsword by a full 20%. So when you do hit...It's usually not actually THAT much better than any other weapon. Despite the effort you put into landing that blow.
I'de say up it to 100% atleast. 10% SWA can go a long way when you're supposed to have a lot of str with it anyway.

Jumping from 90% STR to 110% STR is roughly anywhere from 15-20 SWA depending on the STR you are rocking, while the damage bonus on the Gigantys itself is around 70-80 with the right parts/upgrades/etc. on it, I think your comparison is a little off here, Kingslayer on paper doesn't seem much better than a long sword, except when pitted against 10*s then its true use shines.

Last time when I was hauling around the building sword on a 58 scaled STR character the Gigantys had 135 SWA, I think thats plenty.