To be perfectly honest about the whole RFX75 thing...it just seems to me like a whole lot of trouble for something that can be accomplished much easier. It's going to cost you about the same to just use a regular radio and a small linear amp. Or just have the radio tuned up good and see if that works before installing the linear. The RFX75 is neat, but it's a lot of work, and it's nevery warranteed. I wouldn't want to spend money on something that isn't guaranteed. Also, if you have the RFX75 installed and decide you still want more power, your only option is to step up to an amp that will take 100+ watts of drive, which is big bucks. Personally, I think the sweet spot for $$ spent is somewhere in the 250-350 watt range.
Oh yeah...like others have said...skip the cophase setup. Just run one good antenna.