Oh man, when you said that you wanted good runtimes, I didn't expect you to say 15 minutes. 15 minutes is easy with LiPo. 30-40 minutes is a "good" runtime. With my 8XL setup in my buggy, I get a good 15 minutes on my PolyRC 4400 pack. The 5K pack, should easily get you what you are looking for.

Just keep in mind that you get what you pay for. The MM/8XL is a good system, but heat will still be an issue. It won't be a matter of whether or not your setup will over heat, but a matter of when. This will all depend on how hard you run your setup and the weather. If it is always cool where you are, it may not be a issue at all.

The better motors, like the Neu and Lehner, don't generate nearly as much (actually less than half) as the Feigaos do. By all means, if you can afford it, go with the Quark/Neu 2.5D setup. It's what I'm shooing for, or something very similar. The Mamba Monster Max controller is supposed to be out soon too, but I wouldn't hold my breathe. Mike has told me that he has literally run his Quark/Neu setup for hours without any thermal issues at all. As much of a bang for the buck as the Feigao motors are, they can't do this. Really, by the time I drain an 8Ah pack, I need to let the motor cool off some, before I get back to it. We are running 30 minute mains down here, and when it gets hot and humid, I can't run for that long.

The reason I mention the Quark is because the Neu motors are reputed for not playing nicely with the MM controller, or should I say that the MM doesn't play nicely with the Neu motors (4 pole motors) and the Lehners (2 pole motors) are not available right now. Besides, since you are starting off new with the Lipos, you could just as easily get into a 5S setup, since the Quark will do 5S. Then you could just get an even lower kv motor and run that on the 5S for even cooler and more efficient running. I just switched to a 5S setup and the first thing I noticed is how much cooler everything runs. The motor and esc.