I feel that the questions in your post #1 are essentially unanswered.
1. Yes the RC circuit will affect the rise time if the circuit is operated with a respective fast control voltage. Obviously the circuit is designed for static constant current, so it doesn't matter here. The purpose of the RC circuit is loop compensation. Without it, the feedback loop could become unstable due to the large MOSFET input capacitance. The right dimensioning depends on the OP speed, a 100 pF - 1 nF capacitor should be sufficient for general purpose OP with MHz bandwidth.
2. The current source gain is only set by the shunt resistor, Iout = Vin/Rshunt
Having a small shunt resistor value is reasonable to reduce the shunt power rating, if you want to control the circuit with a DAC, you'll probably keep a voltage divider in front of the OP.