Hi Anthony,
I had a batch of new circuit boards made up following the reference design exactly, with the exception of the switching FET. Again, I used my model instead of the one in the reference design because of it's lower cost and smaller package. As you said, the gate charge and other characteristics are similar. However, I am still experiencing the same problem. At roughly 2 amps, the circuit becomes audibly noisy and the voltage sags. Around 2.3 amps, it goes into over-current mode. I tried further reducing the current sense resistor, increasing output and input capacitance with both low ESR aluminum caps and ceramic caps, etc. Nothing made a huge change in performance. I also decreased R3 from the specified 137k to 51.1k (the next closest value in the same size package I had on hand). It prevents the device from going into an over-current shutdown mode, but the voltage sags all the way down to the 22V in order to maintain roughly 2.2 amps.
Also, I have tried a variety of different inductors from 15 uH to 22 uH without any improvement. (I was concerned that my original inductor was saturating but that doesn't seem to be the case).
Everything works great for lower power situations, but the circuit will be required to occasionally output up to 3.8 amps for short durations. Needless to say, I'm feeling a little disenchanted with my experiences designing DC:DC boost circuits. Any advice?
Thanks!