Bit late with this, but I changed the 40mm tube for 50mm of the same tube.
No noticeable difference in performance... but a definite difference in install effort!
THIS is the pond hose that I used due to the smooth slippery inner wall (thus less drag/turbulence).
Well, so say it put up a fight is an understatement!
Firstly it was clamped to the outside of the connecting point on the bottom of the airbox (after some shaving), and then clamped to the outside of the CAI funnel, which was fine.
Finding the optimum length of 50mm hose which would allow me to place the funnel in the gab in the front bar WITHOUT popping the damn airbox out of it's mounts was another thing altogether.
Even when I cut the hose to the right length, it had to be wrestled into place.
Still pushes against the front bar!
My recommendation is to stick with 40mm if you are using that pond hose. No performance gains from 40mm to 50mm.