As long as you have 510 attys and carts, you're best 'upgrade' would be to go to the eGo or as Michelle suggests, the Riva. The Tornado from TW is another option. All you need is two batteries, a cone and a quick charger to start.
Cignot your Midwest Electronic Cigarette store.
The other new development is the mega eGo cartomizers:
eGo cartomizer
... that have lasted me 5 hours of vaping per refill. And the mega eGo atomizer and cartridges, which also last at least 5 hours. The carto and carts hold 2.5ml of ejuice.
eGo cylinder mega atomizer
eGo cylinder mega cartridge
There are other places to get these but cignot currently has all in stock and is a good vendor.
The Riva battery which is 750mAh rather than the eGo 650mAh - another hour basically, is fully compatible with all above - not all the non-joye eGo clones are - and is available at Nhaler.com and other places.
I started with the 510 and while not abandoning it, I experimented with the KR8/cartomizers and while at the time they were a convenient way to vape for 2 or 3 hours without having to refill, now with the mega eGo cartomizers which go even longer, I find no need for the KR8's any longer. The vapor was always better on the 510 and I found that many of the KR8 cartomizers had an underflavor that was off putting, but not everyone tastes that - many do though. It's not a terrible ecig by any stretch, just that with the eGo/Tornado/Riva that you get the battery life with the 510 and you still have the best atomizer on the market for vapor, imo.