NO!! If you look at the auction, it says this pack has 18ga. wire... That's WAY too thin. Even if the wire was thicker, I would not trust these "new Chinese cells"... Go for something with a reputation, like Hyperion, Polyquest, or LiMn.

20C is unnecessary IMO. If you get a pack that can handle about 45amps continuous with 80amps burst then that should be adequate. A 6000mah 8C pack is rated to deliver 48amps continuously (for example), and would give better runtime than a 4000mah pack. Unless you are extremely worried about weight, I wouldn't even get a 20C pack. You pay a lot extra for them, and 10C-15C packs are plenty...

Sorry I posted the wrong link. These packs are 20c.

http://www.pfmdistribution.com/

The 9xl with a hi gearing for a speed run can suck some amps. It will thermal a 18.97 schulze on a warm day. So I would get the 20c rating.

I am curious, do you think a 20C 2000mah pack would work? It's 20C, so it must work...

The "C" in a rating stands for Capacity. IMO, that's the most important part of a rating. One could get a 4000mah 10C pack that would handle just as many amps as a 2000mah 20C pack, but give double the runtime.
It's up to the buyer of a pack to go for what he/she wants. With a high "C" rating, you can get a smaller capacity (lighter) pack that can handle a lot of amps, or with a higher capacity (heavier) pack you can get a longer runtime. coolhand, I know you, and you love runtime...
Then of course there is the option to get a large 20C pack, but most likely that will just add a lot of weight over the lower "C" rated pack. Not only that, it will also cost a lot more.

Well for me metal man a 4000 mah pack at 15c I am not sure I would run on a speed run with a xl motor for the fact is they can suck some serious amps. The 20 c would hold a higher voltage as well to. Yes I do like a good runtime. I am also a fan of going fast. Now if you talking a 6000mah pack with a 15 c rating I would say yeah.

A 6s 6000mah 15C pack would be quite expensive! I myself can hardly afford a 4s 5400mah 8C pack... In this case it does come down to the budget, and desired weight.

A 6s 4000mah 15C pack could dish out 60amps continuously, for an average continuous power output of about 1200watts (depending on the particular cells), and should be capable of 10sec. 20C bursts, which gives about 1500watts burst. More could be attained in peak draws, I'm guessing about 1800watts. Even this pack would be rather expensive, I'd estimate $300-$315... That's a lot! About 3 times more than I could spend.

Yeah I hear you metal man. I guess it would depend on what you planning on the packs to. It you go conservative it would be fine. I wouldn't worry. On speed runs and such is where the concern would be. I was thinking the 20 c would be better for run cooler than the 15 c and run a higher voltage. I have not ran them yet metal man. I have been looking and listening to you. :D You the lipo man.