Use of Shimano long-reach calipers with "New Super SLR" brake levers/shifters So my bike has Shimano Tiagra ST-4600/4603 brifters (it's a triple) but is fitted with long-drop BR-R451 calipers. In theory, these should be incompatible because of the change in cable pull ratio, which Shimano seem to call "New Super SLR" that happened in the 4500->4600 / 5600->5700 etc. transition.

However, I have found conflicting sources as to whether this is OK, including the Shimano compatibility chart at hosted at Celebrazio which firmly says "No!" (although if you're using the flat-bar lever it's OK?); whereas the Shimano Line-Up chart seems to think it's all right to use as an alternative to the short-reach in-series caliper. Also, a previous edition of the compatibility chart says that mixing the older brakes with newer levers is permissible.

I presume this is the same situation as using the short-reach brakes from a previous series of the groupset with the newer levers, in which case is there any solid advice about making the setup work?
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.