Originally Posted by Dave Wilshere

When you have a large difference in the two tube lengths you don’t get a clean exit of the gasses. It’s a small difference, but it is there.
My first Bandit ended up with a huge difference between the two pipes and it only came alive when I changed the inner tube to one 12mm longer.

Originally Posted by Rudeboy

If you have too much overhang, it will start to move huge amounts of air between the inner and outer tube.

A guy in Germany does a lot of testing and makes YT videos about it.
So he tried to measure the airflow in a double wall tube: with a 10mm overhang at the back, there was just over 40 km/h airflow between the tubes. With an overhang of 20mm, this jumped to over 110 km/h (this is quite a lot of air being moved).

It won't get hot though, not at that short distance. The inner hot flow will not mix with the outer flow enough in that distance.