In relation to aftermarket ecu's on the Focus RS, so far the only standalone unit that has been proven to work with the FRS is the Autronic SM4 - Rob @ SCC / Mike Gurney have mapped a few now & Mark Shead @ MAD has also done a couple FRS on Autronic as well i'm sure

A guy on the RSOC tried both Omex & Pectel on his FRS & neither could be wired into the car properly & he eventually changed to SM4 as well

With the Autronic sm4 i've got on mine, the standard factory ecu's are still retained on the car to control things like the alarm, but everything related to the engine management, etc, is run off the sm4

At the original poster, you really do want to think hard before going standalone ecu on the FRS as you're looking at the best part of £3000 for the unit, fitting, etc, plus you have to put together your own custom wiring loom from scratch so it's definately not "plug & play"