With belt overhaul disabled yellow underground belts have underground length of 2 instead of 4, inheriting the no-longer-extant basic (grey) underground belts' length. That gimps them more than a little.

check the rest of your settings. I'm fairly sure that you don't need the belt overhaul on to be able to use the belt distance overhaul, it just shifts all the tiers, so that yellow becomes T1 instead of grey.
if belt distance overhaul is enabled, try disabling that too and see if it helps.

Huh, I may have misunderstood how the settings work. Thank you, I'll see if I can get it working as I wish it to.