If you want an all around bike with good low-mid...try a set of pt mids, then next in line I would say dmc's or fatty's. For the price I would go with fatty's, they have good low end and a solid midrange and still rev out nicely...remeber for mx you want a pipe with midrange not top end..
About the mx track...its not just the low end that banshee's have a problem with it's the outdated suspension. It hasn't changed since 1987 so you have to make the best of it. I'm now saying you CANT ride a banshee on a mx track it's just you have to work harder. But if you invest in some aftermarket suspension it wont be as hard. If your just messing around on a track then I wouldn't see the need for suspension just yet, but trust me you will love if when your bike handles much better and takes those jumps like a champ. I've riden an axis suspension setup, and that thing rode like a cadilac...just my 2 cents...