Not looking at your DPS, but rather your +spell power...

He said his toons are at +4300 spell power.
Demonic Pact gets your party/raid, a 10% (of your spell power) increase.
So in his case, DP grants +430 spell power, as opposed to +280 spell power from just ToW.
Chances are 1x ToW is up anyway, for the increased crit chance.

In my warlocks case, DP is not even close to the spell power from ToW.
So it varies with gear.

Even if you were to take a 1k DPS hit on the warlock.
If your other DPS were to gain more then 1k DPS across them, it would be a good trade.
But, you'd need a high enough SP on the warlock for DP to pass ToW by a large enough margin.