Level 2. Covers all the basic info you'd normally want.
I'd always used Crystal for setup which is no good now so never really worried about feedback for individual speaker levels for instance. I've heard that higher levels can bog things down a bit but never tried.
One of the quirks of the feedback string is no end of line or checksum so you really don't know if there's been an interruption of any sort. Just parse out the bits you want and hope. Hasn't been an issue so far but might be if you went to level 4 say. Then grabbing the 40th word in the string may be a little more quirky.
At least the CB IIIHD has a fixed response. Something the Compli Blu could use. As it is you don't always know what it meant if the response was asynchronous. Something Oppo fixed in their latter firmware with the verbose mode option.