epc shows differant bushes per model? Interesting.
General thinking on here seems to be that only shocks and springs change.(although have seen mention of roll bars and stearing rod being differant) Did they give you, or can you get, exact details? Part no.s etc?
But for your issue, give them your reg and chassis number and they should be able to give correct items for your car. I would not be concerned with any fit issues as omega suspension parts dont vary in dimensions as such, ride hight and ride quality aside. If the rear donut bushes vary in anyway i would think it would be in consistency of the rubber used or size of void holes rather than their actual size hm respect of ride quality. Forget any issues with carlton bushes imho, they are not designed for the omega, so no reason to "expect" them to fit. (for info, they(carlton bushes) are set on the spacer at a differant hight, so will fit, but then work out after a day or 2)
Edit, re read, sorry, may be worth a try. Not aware of anyone trying firmer "omega" bushes, as not aware of any differances between models, afaik.