Yea, the stock rechargeable pack is ten Sub-A cells on a GT. Part of that reason is they are only 1000ma. Had they used MODERN nimh technology, meaning a 2500ma or higher cell, I bet they could have used 8 instead. So, if you are buying Sub-As then use ten, but if you are using regular sized AAs I'd use 8 so they'll fit and go 2500ma or higher. I believe all the GT packs are Nimhs where as some older Sovereigns used Nicads. Not only that, but I think some of the older Sovereigns used Sub-C cells, which are much bigger than even Sub-As. Make sure you know which is which when ordering if going that route. An alternative would be to use AAA rechargables. Main thing is if you plan to use the stock charger and charge port then you must replace them with the right type- nimh or nicad. They can be different size cells (AAAs, AAs, Sub-Whatevers), and they can have a much larger capacity, but they should be the same type that you are replacing...either nimh or nicad. That little circuit inside the battery pack where the charger plugs into is I think a primative charge circuit, so that's why it's important to match cell type. If you are using an aftermarket charger you could put either a nimh or nicad in there and then hitch your charger up to the larger battery leads (on a GT pack), or take out the little charge circuit and still charge via that connector. Not sure about the details of older Sovereigns. Mainly I'm talking GT here.
A good high capacity nimh 2500ma or higher is a great option, and if not using Sub-As or Sub-Cs they'd be much lighter than what's in there now. However, if you go lipo it's even lighter.
Whoever was asking about lipos. Dig up the huge Excal Battery thread and Kered posted some good pics of his lipo in his. I think he also posted it in the Sovereign GT Mods thread in the modifications forum. I never took pics of my lipo install but he did a great job. I think it was on the same Sovereign model too.