The settings option of the foo_mpcenc.dll plugin contains only the quality sliderbar and makes no mention of the --xlevel swtich. I want to think that the plugin sends the --xlevel switch to mppenc.exe automatically; even though, AFAIK, mppenc.exe itself does not use it by default.
I don't think Case was too sloppy not to use the --xlevel switch with his plugin and not include the option in the settings (or at least point it out), so this is more to ask for confirmation to definitely squash the uncertainty bug.

Yes, it does by default!

edit: why haven't you tried to Search for --xlevel?

I *did* use the search function, although not with a query as simple as "xlevel" but rather more like "xlevel AND musepack AND plugin", and all I got was a thread that about foo_clienc with an answer that wasn't really clear whether it dealt with the foo_mpcenc plugin or rather mppenc.exe. I just did the query with just xlevel and the answer by kode54 popped out alright. I apologize for this, thank you, anyway... Sometimes simpler is better.

Right!
"--xlevel" in foobar2000 forums gives 2 posts asking the same question: one from me 4 months ago and another one month later...maybe other answers around...