As an OT, rotating SE/SP/BB, without either debuff falling off, would require a little over 461(2.38 sec gcd) skill speed. Rotating all three as MT would require more than that accounting for IB. This is from a few minutes on a training dummy.

Overall, IMO, it's not worth it to even try and itemize skill speed, you lose too many other valuable stats. Without any skill speed, as OT your debuffs would be down for about 1 gcd.

I don't think it's even probable to weave all three combos in as MT, considering the loss of threat from fewer BB combos.