IIRC XXHighEnd uses Level 6 by standard (in the Copy to FLAC functionality). To me this seemed the most efficient option (regarding the time it consumes and what will be achieved for compression factor).
-lossless uncompressed (?). (it seems redundant to me, is it the same as using a wav file?).
For adding Embedded Coverart perhaps ?
Do you use the verify option? (verifies compressed audio by reading back from hard disk).
XXHighEnd itself does not. Looks stupid to me anyway (I'm not veryfying my Word documents either). However, there's another kind of check which will fail when something went wrong at the encoding. Maybe that is what you mean (this is not part of standard FLAC and is an external program).
HTH
Peter