It depends and honestly, I think some feet are much rarer than others, though SE will say the drop rate is equal. I have a lot of experience farming big gold chests for KIs and am only just getting to grips with maximising the potential to farm boots. Seven hours soloing as DNC for my Charis toeshoes last weekend (with no result) actually taught me quite a bit ... indirectly.

High amber does not make more or less feet drop - it only seems that way. They have a constant rate of dropping. High amber adds items to the roll of things you can find in the chests, like above post said. Though my opinion differs slightly, and I'd say at capped amber with big golds you'll get one of the below:

a) Key item for NMs.
b) Revitalizer.
c) Megalixir.
d) Collection of one to four high-amber temps like Fanatic's powder, Berserker's tonic or Gnostic's drink.
e) Synthesis materials or scrolls or combination of both and random chance of Empyrian boots.

From what I've seen, the strength of amber does not affect which KIs will drop in chests. We've had the rarest Tahrongi KI (Ophanim) drop on only the second big gold, with low-mid amber. Strength of amber does affect augments on equipment such as Diamond rings (typically will only see the +3 INT/MND rings with capped amber).