The Definitive Dark Knight Guide

[Daerthalus]

My theorcrafting has shown that.

Assuming: 2.5s GCD, Dark Side Costs 2.5% MP / 3s

DA+SE Combo costs: 18.75% MP
Delirium Combo recovers: 6.25% MP
No Delirum Combo recovers: 8.33% MP

So the idea that 2 (Heavy Slash+Syphon Strikes+Delirium) = 1 DA+SE, is false.

It actually takes 3, since Dark Side keeps draining your MP during the time.

At 2.5s GCD you lose 2.083% MP every GCD.
A Scourge actually costs you 2.083% MP.

Dark Side basically adds a 2.5% MP * GCD time /3 cost to every single action you take.
The faster your GCD the less expensive a non syphon strike combo action becomes, and the more mana you regenerate from any comboes involving Syphon Strike.

At 2.0s GCD:
a DA+SE Combo now costs only: 17.5% MP
a Delirium Combo recovers: 7.5% MP
a No-Delirum Combo recovers: 9.167% MP