They do have lots of Ah's but of course it takes two before you can compare those Ah's to a 12V battery. In terms of energy density by weight you can compare two 60lb batteries on Deka's website:
21.35 Wh/lb for the Group 31 (59 lbs, 105ah)
21.32 Wh/lb for the Golf Cart GC10 (60.5 lbs, 215ah)
Basically a rounding error in terms of the difference. I've run the numbers on density by volume and again it comes out the same. So there may be advantages for the GC's but it's not density.
Not sure what this means.
Of course you need two 6- volts to get 12 volt current. The advantage of them is that the footprint for the 2- 6 volt is almost the same as 1 12 volt. So for the same footprint you get twice the ah. NOt sure what density has to do with it. The advantage is more for the SIZE...irrespective of weight.