Disc wheel v zipp 808 firecrest

Would a zipp super 9 be much faster than a zipp firecrest 808 rear on a TT canyon speedmax. Would be using it for 10 and 25 miles races.

I would say 15-30 seconds at most over a 40km TT. If you already have the 808, then just buy an aerojacket wheel cover: http://www.wheelbuilder.com/aerodisc.html

What sort of times are you doing?

Yes quicker, but not by much, plus it sounds great