If you were used to the linked brakes, you will need to recalibrate yourself on separate rear brake. With a separate rear, you get to decide when you want rear brakes and when you dont. I have the exact same rear caliper setup with a RC51 master. (check my gallery) I believe the CBR600RR, F4i, and RC51 are all 14mm bore M/C's. I once calculated that with a 14mm m/c it gives a set up very simular to a stock 4th gen rear brake performance. -- If you decide over time that it's still too sensitive for you, I also calculated that a 5/8" bore m/c would tone it down a bit and still be an acceptable rear brake. Unfortunately, I cant give a specific bike or part that has one.
Another tip - I believe that 2005 VTR1000 super hawk clutch master is separate resourvoiur type and may be another option for you, maybe cheaper.
BTW awesome job - good luck