I shoot Beman ICS Bowhunter 500s. I also found the GTs to be stiff. I use either Zwickey Deltas with a 100 grain steel adapter or 160 Snuffers with a 75 grain adapter. Moving the weight to the front helped both FOC and spine for me. The brass weights work, but not as well.
I also draw right at 28". Here is the setup I use:
ARROW length 28.5
shaft+feathers+nock wt 245
separate insert weight? 30
adapter weight? 100
point weight 135
total arrow weight 510
balance length 21.00
FOC % 23.68
draw wt 52
arrow wt 510
GPP 9.81
Kanatis are pretty forgiving of spine as a rule. I shoot the same thing from my 46# as well and notice no difference. I killed a nice 8 point buck with it last fall (complete pass-through at 22 yards).
I also use this setup with my Orion 53# and it seems to like them too. Lately the only trouble I've been having is with Robin Hoods
My son used 1916s with 150 grain Woodsman broadheads out of his 51# 58" Kanati last fall and shot through a nice 12 point buck. I think he had them cut to 28.5", but I'm not certain of that. He probably draws 27 to 27 1/2".
Welcome to the Kanati brotherhood! My 46# Kanati is my all-time favorite and my go-to bow.