The immolate change is big for demo, no denying, and at first glance it does look like a good trade to drop improved demonic tactics for aftermath to increase immolate's periodic damage. IDT already serves a very small role in demonic pact uptime, and that role will be effectively ending in 3.3.3. The damage from IDT isn't that impressive...
However, a look at immolate as demo isn't heartening either. Demo is a very nuke-favored spec, it gets even less direct benefit from it's DOT effects than destro. Corruption's role is as a proc generator, and immolate's DPCT is propped up primarily by it's very long duration and locked in by 4pt10. Ënmity of Mal'ganis, among others, have already tested the change, and despite the huge buff to immolate, demo's immolate is just too weak for Aftermath to pay off, even against the small benefit of IDT, changing the spec for aftermath works out to a loss of over 100 dps in helter skelter mode, higher yet in patchwerk mode.
EJ's current round of tests agree, and have some other interesting notes. While Destruction's buff is also huge, it's still hampered by it's poor crit and haste scaling. It performs very strongly in T9 gear profiles, beating out affliction and demo, but falls behind even demo in T10 profiles. Right now, what destro needs doesn't look to be simple damage buffs - the things that will keep it up there in T10 gear will disproportionately buff it in lower gear. Cataclysm will require some very wide spread work to address haste and crit difficulties suffered by a number of specs, not just destro. The mastery system may be a strong route of this - for example, propping up a spec's crit scaling is often as simple as increasing it's crit bonus. Some of the mastery previews included crit bonus as a mastery spec effect.