It seems that this mod doesn't fully work with Double Speed Belts, although it's specified it does.
Vanilla Express belts are 'upgraded' to 60 items/s by Double Speed Belts, but this mod's underground connectors are still only at 45/s.
'Normal' Ultra Fast belts from Ultimate Belts are upgraded to 480/s, but this mod limits the underground connectors to 90/s. (The speed is this cause I am using bob's logistics which adds another 2 tiers after express which are upgraded to 120/s and 240/s respectively, and their underground connectors aren't updated either, so they are capped to 60/s and 75/s.) All tiers below express work like intended though.
Let me know if you want to see my mod list.