In building an electronic load with a mosfet and op amp. It adds some benefit to use a mosfet driver IC?
Vgs =5V barely satisfies nominal FETs and not worst case to load with 5A.

Use 6V+ unregulated to power OA and Vds*Id * t * f * Rja will be your junction temp rise with t * f = duty cycle. You can then calibrate pot to go from 0 to 10% Imax or 10% to 50% to 100% with an input waveform. I would use an NPN open collector on OPAMP (Vin (+) to switch off current with any duty cycle form 0 to 100% for all your load regulation tests and use 50 ohm AC coupled cable and 50 Ohm terminator to measure ripple with 200MHz BW. 

Then examine DCR, RdsOn, Cap ESR ratios to understand step voltage transient errors and consider what ESR and damping factor with compensation Lag/lead filter gives you the acceptable voltage error for your operating load requirements.