at our track (
www.michianarc.com - 36x60 track size) we mix 17.5 and 27t in the 12th scale class. I think the 27t is just a hair faster, but both "parties" would have to drive perfect lines to see it.
That being said...
in our truck class (stadium trucks on carpted "offroad" track w/jumps and bumps) there is very little difference between 13.5 and 27T.
And that being said..
when we go outside to larger tracks 13.5 beats 27T all day long. I personally, havn't tested 17.5 to 27T on a larger track to see what the difference would be.