[lysace]

How much overhead (in terms of e.g. transistor count or silicon space) does this add? Surely at most it's a single digit percentage?

[immibis]

There isn't a separate 8086 core in every x64 core. The whole core has "if/then" spaghetti logic scattered throughout to alter its behaviour based on being in 16-bit mode.

At best, they might have been able to confine the needed logic patches to the instruction-decoding front end.