Hi MA Pete,
Ummmm.... a number of people have indicated that the '07 Apollo "needed" the 75 cm fin.
(Yes, I hope you have alot of water depth as that's a really big fin with a fairly special "best at low speeds" foil) to achieve it's "earliest planing" status.
These same sailors were disappointed with the '07 Apollo with 70 cm race fin like your Deb R-13 as the board didn't then plane as early, and the smaller 70 cm fin did't give the Apollo full formula like characteristics in 10 knots and up.
The new '08 Apollo has addressed these issues, but I think it still needs the really big 75 cm fin to attain the best early planing (which is supposed to be even earlier than the '07 board).
I don't have the new Apollo and probably won't be able to get one, but we'll see and feedback from those of you fortunate enough to have ordered one will be really valueable!
Hope this helps,