Having been to the RCR facility and ordered an SL-C, My impression as an engineer was that of "why has not anyone done this before". The GT-40's from RCR are indeed well thought out, well engineered, and the updated suspension is superior.
The fit and finish of the gelcoat bodywork is one of the best around. I have looked at SPF, etc. Performance comparable to the Ultima GTR is attainable within the aero limits of the GT-40,s shape. For value, completeness and support I believe that RCR is the way to go. As for repairability of the RCR mono chassis structure, it utilizes easily obtainable shapes and plate and (in my opinion) is relatively simple to repair. Of course, my first advice would be to see as many GT-40's as you can, drive them if possible and make a decision and commitment. It will be your baby so go with whichever pleases you most.