@PoP: Not as far as I remember, but I may be wrong

.
To recap a bit, because I've been poring over this all day (funnily enough, nearly the exact same query came up on IRC ^^).
* A combo file straight from KindleGen => "Personal" banner, crappy small embedded thumbnail.
* EXTH 501 set to EBOK => No banner. Crappy small embedded thumbnail for KF8/Combo files, possibly slightly less crappy generated thumbnail for M7 files.
* EXTH 501 & EXTH 113 (+/- EXTH 504) => No banner. No thumbnail either, unless the ASIN is a valid ASIN, and your Kindle is connected to the net, in which case it's pulled from Amazon's servers. (Basically, that's how a file you get straight from the KS/Cloud behaves).
* Now, if you sideload this last kind of file via Calibre's "Send To Device" feature => No banner, and a pretty thumbnail!
Speaking of Calibre, when you do a Mobi or AZW3 conversion, unless you mess with the settings (the whole FB sync thing, manual PDOC tagging & co), it will generate this last kind of file (ie. the best kind).
There are also a number of codepaths in Calibre that don't really try that hard (or at all) at distinguishing combo files (for example, because that's what I was looking at, and it's partly my fault, the APNX sideloading handles combo files from KG as if it were a M7 file, which might not be the best course of action). So, yeah, unless you have a very good reason, I'd do without the combo files, personally

.