You're in a win win situation.
T6 pipes hold their value pretty well, as do T5s. They're probably the best selling pipes of all time...in fact, I think the T5s are.
The bike ported is gonna rip. T6 are a damn fine pipe. If you were going to go drag porting, etc., then I would say skip Toomey all together and get CPIs or Shearers...but, you're going with a dune port.
I'm sure that motor will run just fine, he does nice work.
You'll find most people that say get rid of the T6s and get T5s have had only one, maybe two sets of pipes in their entire life, still running the same motor config, and have never tried T6 back to back with T5s on their given motor...
What it all stems from is this. On Toomey's website (who was bought out, forgot by who) it says T5s are for ported motors, T6s are for stock motors.
Well...Toomey doesn't do porting. Or if he did, it was a LONG time ago... So...that being said, how many different sets of cylinders do you think he tested that out with? Zero.
He probably took a set of stockers and put T5 and T6 pipes on it back to back and dyno'd it. T6 won.
Some guy might have ground up a set of cylinders and gave them to Stuart Toomey to try...they made more power with T5s...so he put that disclaimer on the site.
There are dozens if not hundreds of builders out there. Who's to say the ported cylinders he tried T6s on was worth a shit in the first place.
Go for it...you have very good alternatives if they don't work out. My money says you'll be fine.