I'm hoping to make a high speed printer, definitely above 100 mm/s for pla (thinking 150+). As for flexibles, with some minor modifications to my mk8 style extruder I could get TPU printing at 60-70 mm/s at 0.25 layer height (I'm a very impatient fella haha) so I'm aiming for better than that, at least for my direct drive variant.

But yeah, I agree, at 100mm/s PLA you wouldn't need or possibly even see the difference between single and double sided drive.
This thread has a good comparison photo if you scroll down.
https://ultimaker.com/en/community/1...1#reply-120122