Sorry for the delay. At the moment, I just have stock X-Challenge forks on there. But I as well also just ordered Hyperpro springs. They should arrive next Tuesday or Wednesday. So I have a 2009 X-Co with a 2007 X-Co height shock(brand is TFX), the X-Challenge forks and X-Challenge wheels(21" and 18"). Not sure yet how the new springs will affect the mounted length of the forks, but currently I think the bike geometry is pretty close to the original 2007 X-Co's. Haven't gotten the chance to dirt test it yet and probably won't until after the new springs are on. But have to say, pretty happy with how things are at the moment. Don't know much about the X-Ch air shock. But I would probably say you'd be better off with non-BMW suspension. I would assume any shock made for the X-Challenge would work, but I'm not sure. As for me, being at 5'9" and with baby legs, haha..... as much as I welcome greater ground clearance, I also like still being able to reach the ground. So 2007 X-Co height is fine with me. Maybe one day I won't need the comfort of being able to reach, but for now it helps.