Kered, totally agree with all you've said. I believe the reason why Minelab puts 10 Sub A cells in the Elite/GT is because they are old Sub A nimhs. While Sub A cells are still used a lot in RC these days, 1000ma isn't that high of capacity. Had they provided good 2500ma or higher cells then I would suspect they could have got away with using 8. With that low of capacity the source voltage will drop fairly rapidly, so I think they used 10 to keep it above the LVC (low voltage cut off) for a longer period of time.
Like you said, most of my high quality nimh or nicad cells charge to and hold a much higher voltage than 1.2V per cell. I believe that is more of an industry standard or average in terms of designing products to run on them. I've got 2500ma Energizer 8AA nimhs in my RC radio and it constantly displays the pack voltage in big numbers on the computer screen while flying. So long as I peak them before heading out they stay well above 11V just about all day long. Since I gather the GT's voltage regulator cuts out at around 10.5V or so, a freshly peaked high quality/high capacity pack should probably run the machine for a long day's hunt without issues. As you also said, non-rechargeable store batteries don't offer nearly this much capacity, and as a result the source voltage derived from them will soon be well below that of a high capacity nimh or nicad pack.
In terms of lipos as another example, a 3 cell series pack is often referred to as 11.V volts. In truth they charge to 12.6V. The other neat thing about lipos is they hold their voltage very high all the way until the end. In that respect they can often run a product longer than a higher capacity nimh or nicad because the lipo won't hit LVC until right near the end of it's discharge. For that reason a 1000ma lipo can possibly run a device longer than say a 1500ma nimh or nicad pack, because those cells will hit LVC quicker due to the more constant voltage drop as they discharge. The other perk about lipos is they don't discharge while sitting on the shelf. Charge a pack fully and one year later it's still for the most part fully charged. That's not a good idea, though. If they aren't going to be used for months then put them to a sleep voltage and throw them in the refrigerator. It protects the capacity of the pack over long periods of time of non use.