Have a WC46 that I run on the 2025R with 18 PTO hp. It loafs along; so, I'd say your GC with nearly 2 more hp should be fine with a WC68. Think hp goes up by sqrt of tree diameter.
The only sure chute plugger I found on the WC46 was tiny vines growing in the upper parts of the trees. After I found this out, I'd chop up the bottom of the tree and then reverse it out and make a pile for the bush hog.
Wood chip nature varies considerably with the wood, its type and wetness. We chipped up about 30 FEL sized piles this winter, and the blades are still sharp.