Use of Shimano long-reach calipers with "New Super SLR" brake levers/shifters
I am currently using a mixed set of calipers, a Super SLR BR451 front and a non-Super SLR BR600 rear. I was using an older non-Super SLR Ultegra 6600 levers and the braking was good but not outstanding. I have recently switched to Super SLR 105 5700 STI levers. The front braking is noticeable better. The rear has fine power, but the amount of force needed to actuate the brake has increased. I first suspected poor cable and housing routing, but it was not that. 

I plan to get and try a rear BR650 caliper in the near future.