Absorption damper, size matters
In my experience, for good fuel consumption and performance, you need a can with the largest volume you can find / fit. What also helps is 'headers' that have steps in diameter or, even better, with a cone diverting towards the exit. As you have a XT400 I think the laser produro would be ok. It is not very good on the XT600 but might be ok for the smaller engine, and it has a kind of step / cone in the pipe leading to it.
Make sure to remove the bird gauze at the end as it gives a lot of resistance to the gas and does not really dampen noise.
Normally, no rejetting required as Yamaha's are set too rich from the factory.