MOSFET vs Diode Based Peak Detector Circuits
Using a MOSFET is tricky. For instance if the output has a resistor to ground to slowly discharge the voltage accumulated on the "storage capacitor" AND to counter leakage currents through the MOSFET charging the storage capacitor up you have an op-amp with gain in the feedback loop. That gain is massively non-trivial (i.e. tens or hundreds) and the whole thing turns into an oscillator.

Given also that the parasitic capacitances around a MOSFET are usually an order of magnitude greater than what a simple IN4148 or BAS16 diode would have, the signal-peak capture speed is poor compared to a diode.


  I've found that with the right op amp and MOSFET, the performance of
  the MOSFET circuit is far superior.


I can't imagine a scenario where it would but maybe you can show one?