The other thing is that they drop the voltage quite a bit during the first 24 hours. The best way to use them is straight off the charger and hot. A lot of people charge an Nimh battery maybe the day before they use it, or leave it in a model and use it may be once a week. This will reduce the constant voltage available, and it will appear that the battery is under performing. Changing to Lipo's will appear to offer more power, but some of the difference is down to the way that the two battery types function.
Regarding Nimh batteries, the cells are tested during production. The top performing cells go into the premium batteries that the racing guys use and that cost at the top end of the market. The lower performing cells go into the run of the mill batteries.
6800mah batteries aren't the whole story, what is important is the length of the linear power delivery. Good batteries are designed to drop the voltage when first used after a charge, but then deliver a constant voltage for a fixed duration. It is the length of that fixed duration that determines the quality of the battery. Tank systems have low voltage detection, and that means that if your battery quickly drops to the cut off point...the tank stops.
If you take your tanks away for weekends etc battling, there is the re-charge to consider, and how long it takes. To fully charge a 6800mah battery, even on 4 amps, is likely to take the best part of an hour. A 3-4000mah battery won't last as long, but the charge time may only be 20 minutes.
I used to think batteries were just batteries until I started reading up on them...
RobG
PS - Tamiya do not make batteries, if the seller is mis-describing his product that way to sell it, I would also doubt the other information provided?
.....just remember to do the initial charge with your stock drip charger...then your aftermarket after...initial charge for a 7.2 is anywhere from 4 to 6 hrs 
