We need them
Since increase of belf speed probably won't break existing things, I'm not sure if old belts have to be intact...
Just a speed increase wolud be enough, but if you want you can make a mod startup settings:
- [bool] speedup: original belts will be faster (15x i/s)
- [bool] add: add a copy of belts with different speed (13.3x i/s if "speedup" is active, 15x i/s otherwise )
- [bool] convert: add conversion recipes (stack->stack, 0.1sec or so)

https://github.com/npc-strider/017-science-transition-mod/pull/1

I know that Bob added high tier belts somehow, but on Reddit a dev said that this is stuff is too hardcoded to really mod. So does this really behave the same way 0.17 will?

The developers affected the spacing of items on the transport line from 9px to 8px (This is not moddable afaik), keeping the speed the same. This mod increases the speed of the belt, which therefore increases its throughput to 0.17 levels. The only difference is that lower tier inserters may have trouble grabbing items due to the increased speed, while in 0.17 they shouldn't have any issue.
The mod should work for modded belts as it affects all transport-belt, splitter and underground-belt