I just want to start by saying that I absolutely LOVE this mod, and am on the side that as you go up a tier the belt speed should double.
As such I'm also on the side that Assemblers should also double per tier as well in craft speed: T1 - 0.5, T2 - 1.0, T3 - 2.0.
You can cleanly double the power requirements as well as I would find that balanced.
I hope you can make this mod, I can't seem to find one anywhere that does this, and paired with the Double Speed Belt, it is my new standard for any game, vanilla included. I have no experience in modding sadly.