On the other hand, I did some testing of my own (with the help of our resident Shadow Priest) on the live server (stupid PTR and not letting me log in). Shadow Priests already have their DoTs affected by haste and have a way of renewing it. But because I'm dumb and forgot exactly what I was looking for, we ended up testing something completely different that had odd results given the PTR testing.
The Shadow Priest 2P T9 "increases the duration of Vampiric Touch by 6 seconds" (incidentally, our 2P T9 does the same for Flame Shock). When he had this set bonus activated, he would only ever get 2 extra ticks...no matter how much haste he had. We even Blood Lusted, and still only 2 extra ticks. Obviously, this is counter to what is coming out of the PTR.
I think the difference lies in when the 6 seconds are added. With the 2P T9, the 6 seconds is added prior to spell casting. Because of this, it is also affected by the haste value. That is to say, the duration of the spell may be calculated something like this:
([Original Duration] + [2P T9]) * [Spell Haste]
This can happen because the extra 6 seconds are always there.
But now we add 6 seconds after the fact; and indirectly to boot. The equation becomes:
(([Original Duration] + [2P T9]) * [Spell Haste]) + 6 Seconds
This question remains: Is the extension suppose to be 2 ticks or 6 seconds? Since it's still in the PTR, all we can do is wait and see what Blizzard does.