I think my preferred calibers would be:

9.3x74
.450/.400
.500/.416
.450 No. 2
.577 NE

If pushed came to shove, I'd really be torn between the .500/.416 and the .450 No. 2. I really like the additional velocity of the .500/.416, but the comparative practicality of the .450 No. 2 is very attractive. For a go-for-the-throat stopping rifle, the .577 looks very hard to beat.

Not that the manufacturer has contacted me on the subject.