The G hub extension, which is one inch longer than the stock C hub, with the N7605() blades was configured to be used in conjunction with the D-4798(P) spinner when installed with the stock RV-8 cowling due to the cowling's unique shape in order to prevent cowling interference issues with the wide blades at high pitch. Effectively, the G hub moves the blades forward and then the D-4798(P) negates some of that movement such that the cowling to blade spacing is increased.
The available integral extension needed to mimic a 2.5" extension over stock as specified by Showplanes would be an M extension, i.e. HC-M2YR-1()X/(), which is nominally 2.563" longer than stock.
The C-3533(-1)(P) spinner provides similar spacing between the blades and the aft edge of the spinner as the G hub/D-4798(P) combo at the increased diameter for the Showplanes cowling. I am unsure as to the specific geometry of the Showplanes cowling, and whether this clearance would be adequate.