You will never be able to straight up import an RS...
Crates are inspected at the docks and unless it's listed as a race car (you would need a letter from Ford saying that it is a race built car not intended for public road use) you cannot bring it into the country.
WHAT YOU CAN DO
Option 1:
contact an "RI" (Registered Importer) who can import the car for you and will make modifications to match that of the US spec Focus in terms of EPA emissions standards as well as DOT standards for lighting and crash-testing... this will cost you easily $10K over the base cost of the car. The RI will assign the car a US VIN and for all intents and purposes they were the ones who built the car.
Option 2:
Find someone in the UK who can scour junk yards for you... and ship you all of the necessary drive train parts to convert a US spec Focus to AWD and whatever else..
as long as you don't modify any of the safety features or emission features of the car (this means you're stuck with the US factory engine) you will have a legal focus that is essentially the same as the UK spec RS (and LHD too!)... A lot of people do this. I know a lot of older Impreza owners who imported STi drivetrains from Japan. This is also your cheapest option.
There are other Not-so legal options. A lot of people bring skylines into the US from Canada since they can be easily imported into Canada and it's easier bringing them over the Canadian border than it is from a shipping port. While a Skyline isn't exactly inconspicuous a Focus is... You might be able to find someone in Canada that has already imported one there... I don't even know how you'd get the paperwork squared away though... maybe find someone in Canada who can legally register it for you and and then just drive it around the US with Canadian plates...
alternatively you could bring the car in through Canada and find a US focus with a blown engine and then swap everything off the UK spec car onto the US spec chassis...
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Honestly your best bet is to contact an RI and see what they say... for all I know they've done a few before... keep in mind that ANYTHING is possible... it's just a matter of money