When I researched the F-5xc I had my gf´s F-5xb for comparison. The latter was tight with the mirror box of my Olympus E-3, so I ended up with the F-5xc because of more width.
Luckily the F-5xc proved to be wider and able to accommodate my E-3, even if the actual (I first read the ones saying 5") specs aren´t different. Probably because there´s no inner compartment like there is in the F-5xb.
As for insert for a nose down camera, none of the Domke inserts really fits the bill. I had to go with a Billingham [URL="http://www.billingham.co.uk/acatalog/SuperFlex_Inserts.html"]SuperFlex[/URL] 9-18 for this purpose.