i think your getting confused with the GRRT..... 17.6K is about what the Jen Tunned MVVS 40 gets, tho you never said that...
i run a GRRT in a Magnum and it uses a 8x7 ( only because i can't get the right sized APC down here ) in a Magnum it's fine, but i think it's too highly strung to pull a great draggy delta about.. time will tell.
if i was you I'd crank the engine over and run the pipe INSIDE!!!!! the wing, make the most of that thick section. should also hide the engine well if you get it right.. be easier to do all this on the old kit, but the new ARTF a nightmare.

if it's still there look at my Magnum review at www.philsrcworld.fsnet.co.uk