Each next level should be in 2 times higher than the next
Or maybe for better balancing - each next level should be in 1,5 times higher than the next

So it'll look like
1) Basic transport belt - 10 items per second
2) Transport belt - 20 items per second
3) Fast transport belt - 40 items per second
4) Express transport belt - 80 items per second
5) Turbo transport belt - 160 items per second

And thanks for mod

Hmm 160 items per second is crazy - even turbo inserter hardly time to grab items and sometimes he's missing
May be 1,5 progresion will be better

Nope. That would result in the belts being too overpowered. If you noticed there used to be a level of belt above purple (green) which was way too fast for the game engine to be able to handle. It was really buggy caused graphical issues and I believe added unneeded lag to the game as it tried to accelerate and collide items at such a high speed. And people always think about belt speeds the wrong way. Yellow belt is 800 items per minute, red 1600, blue, 3200 and purple 6400.