Requires DHooks 2.2.0 Detour
Simply put, set "sm_tf2_maxspeed" to whatever value you want (within reason) and that will be the new speed limit. There are some quirks, like forcing their speed with the max speed netprop won't take effect unless you disable prediction on them. But attributes and some other natural speed modifiers will work just fine without screwing with prediction
There's the command "sm_setspeed", which defaults to "cheats" flag, which given a number will add a speed attribute of just the right value to reach the desired speed (desired speed / current max speed = multiplier to reach it). No arguments provided resets it.