I did a couple of recordings with Zero Boy, who is known as a live performer who uses vocalized sound effects. The project involved sections of James Joyce's Ulysses, which ZB read and provided sound effects for. As the point was what he brought as an artist to the reading, I did not modify any of his sounds, though I did layer them (he provide various elements for me to layer) and mix them appropriately. The recording was done in a voiceover studio with a U87 to Pro Tools.
Based on this experience, I would say that vocalized effects are their own category and will not sound the same as typical FX recorded in the field. This may or may not be appropriate, depending on the project.
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No experience working with an expert (which makes me think of Michael Winslow), but I'll occasionally record myself if I can generate the specific character of sound I'm looking for. I find that vocalized sound effects can be extremely useful as a base layer in designed effects.
One of the competitions that ran when I was still hosting the Sound Design Challenge was the creation of creature vocal effects using only the human voice as elements (processing was allowed, but no layering of animal/mechanical/other sounds). There were a lot of unique entries. You can listen to the finalists here, or listen to all of the entries here.
Having access to someone like that would certainly be a privilege! Like Shawn, I tend to us my voice as a base layer especially when creating actual creature sounds or animals. I may eventually omit them from the final mix, but to get the initial intensity, tone and direction it's much easier for me this way. I then gather material and build layers until I feel it's sitting right.
If the project would call for a pro, or a pro can provide the sound you're looking for, i'd definitely push in that direction ;)