Just FYI, I had an SVT for 6.5 years or so, and I never could get comfortable in those seats. So I'm not 100% sold that buying SVT seats would solve your problem. I had the same problem (back pain) and also I seem to recall sometimes leg numbness/tingling. I tried every conceivable positioning of the seat and was never satisfied.
I just got a 2010 Transit Connect as a work vehicle (which is based on the same C170 platform as the Focus) and the seats and seating position seem to be very similar... unfortunately. Maybe it is something inherent in the seat or the relation of seat to car. FWIW I'm about the same size (5'9" and 185 lbs). Good luck finding a solution to this.