Item Bonus Speed is buggy when it comes to value, the next level (2,3,4, and so on) values will not be used unless you use tricks that BonusMod uses.
Blame Blizzard for this bug.
You can utilize Bloodlust, which AFAIK doesn't bug. Just use the same check but order the unit to use Bloodlust (set time to -0.01 (?) or 99999.00(?)) and if you don't wanna, try to use hidden buffs, there are some way AFAIR.