Thanks everyone for the replies. I just got back from 1500nm trip into severe head winds (40knots on the ground) so I ran things pretty hard and for once I never had to worry about cylinder temps. And yes it was that piece of metal that ramped up the front of the cylinder, not the air scoop for the airbox. Pretty consistent now running at 75%, 2500MSL, 24c oat #1 361, #2 360, #3 379, #4 390, #5 379, #6 398. I found a few gaps in the felt around the engine mounts on the left side, so plugging them up will be my next job but I'm just happy I can keep the temps under 400 now.
Thanks