[nitrogen]

Only the two most recent overclockable models (Devils Canyon variant of Haswell) have VT-d...

This means that previous *-K (multiplier-unlocked) models have VT-d disabled, right? So you have to choose between overclocking and VT-d.

[wtallis]

Yes, although prior to Haswell it was still possible to do a moderate amount of overclocking on the non-K models that do have VT-d; the multiplier could be increased by about the same margin as the Turbo Boost feature uses, and you still got Turbo on top of that. Haswell marked a shift from a limited-unlocked multiplier to a fully-locked multiplier for the non-K parts.