The pipes aren't really any good on either the Yam or the Honda. Both are too small and poorly tuned. Both can be improved a LOT. The Honda can handle a 40mm carb, when used with a hc piston, porting and one of the mild cams that are available. Also 1 1/4" head pipes joining into a 1 3/4" collector and then a meg. will help both engines. I was told a long time ago that the XT/TT350 MAY be able to use the XT550 carbs. Porting would help the 350 a bunch, along with some os valves if they will fit. Basically, the 350s are nice play-bike engines, and don't make any serious performance. Years ago, I built similar complete exhaust systems for both of the engines at the same time and the Honda with the larger valves would rev out strong. The Yam would pull the front wheel up with a simple twist of the throttle, but flattened out in the upper rpms. We continued to modify the Honda, but the Yamaha owner didn't want to pay the bucks to get more. This was in the early '90s.
Swiss