I just looked into Railcrafts Source after fixing it myself and Slopes in Railcraft have a max speed of 0.45, which is only a TINY bit faster than Vanillas 0.39, High Speed Rails on the flat Floor have a max Speed of 1.1.
This is roughly 2.75x faster than Vanilla Rails, … and makes them faster than my fastest rails, which are 2.5x faster… (I guess I need to buff that to 3.0x)
In order for Railcraft Rails to allow a high Speed, the next Rail needs to have the same orientation as the Rail the Cart is currently on, and both Rails need to be Flat.
This prevents the Bug where at 2.5x the speed or higher speeds, the Cart crashes into Slopes, before it even is on the sloped Track itself.
I somehow managed to GUESS the same Solution Railcraft has, and made it so that for my High Speed Stuff you also need a flat straight Track of 3 Rails (one in front, one behind), and otherwise the Cart will cap at normal Vanilla Speed.