I agree with dudley about the funnel efect of the butt guide. I had a 14 and 10 fuji nsg on a 8wt that I built back in 1989 and occasionaly I would have a problem with the butt gude. In my experience with this size guide any twist or loop in your flyline gets compressed too fast and tightens as it passes through a 14 butt guide limiting your cast. Again, in my experience with a larger butt guide the line flows better but I also only fish the salt with stiff bonefish tapers so your mileage may vary.
I know 20 size guide sounds big but that is the outside diameter the actual inside is 14 and a 14 has a inside dia. of 8.
Hope this helps, best of luck with your rod, Jack