Most Omega ADs in the uk will see you getting up to 20% off list if you have half decent negotiating skills, so you may want to consider that avenue first.
If it was a rolex stainless sports model then very little-if any- negotiation to be had, but omega has more wiggle room.

Also, dont forget that US prices are quoted excluding sales tax, so you would have to add that on too if buying stateside. Generally at the moment there isn't a huge saving to be had buying most decent watches in the US new- it was great when the exchange rate was $2/£1 though!

Warranty is worldwide wherever you buy, so dont worry about that.