I have run a Venom 4kMaH 20C, a Gens Ace 4kMaH 30C, and a SMC C-Max 5400MaH 50C, all in a Spec (aka, box stock) Slash.
The Venom had decent power, but was obviously lacking against the higher C packs, some trouble clearing the big double at our indoor track if your line was not dead on.
New layout every week, but you can see what I mean in this vid. 2:40 or so to 5 min is spec slash, the bright yellow truck is me. You can see a few times where I fell short and landed on the top of the double, rather than clearing it.
With the Gens Ace, the higher C rating gave me a bit more punch, and I was able to clear the double with relative ease.
The SMC is a whole nother animal. Even if I screw the line completely, I can still muster the power to clear the double, but I also have more punch than is needed elsewhere, so I have to restrain my throttle on smaller jumps (which I'm not too good at).
All 3 packs have been trouble free, and I've had the Venom the longest. Of the 3, the Gens Ace was the cheapest, when ordered with other items to hit the free shipping point. With shipping, I think it might be about equal to the Venom was at the time. The SMC was bought (is that right? Doesn't seem right...) locally for $50, as I recall, which is only $10-15 more than what I payed for the others, but is a much better battery (in my experience).
Both the Turnigy/NanoTech and Gens Ace packs vary quite a bit depending on which pack you get, if they're good for their rating or not. I still use the 6600 nano's for my 4wd open sc though most of the time.
"We do not act rightly because we have virtue or excellence, but we rather have those because we have acted rightly. We are what we repeatedly do. Excellence, then, is not an act but a habit." -Aristotle