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.
Trevor Parker, Engineer, Hartzell Propeller
Opinions, information, and comments are my own except as noted. Guidance provided here may or may not be applicable to your specific installation.