yes

That's a lot! Normally 14 cells is considered the maximum to run on the 9L.

But if you are fixed on using that much power, then 5s will be fine. That would be more like 17cells though, with a lot less weight. 6s would be like 20cells. I would strongly reccommend that you use 4s on the 9L to keep the temps. down, or just go with a 10L for use with 5s Lipo.

OH sorry, Dont know what I was thinking!!! I have a 7XL. I am so used to reading in the forums about 9L, it just stuck when I was writting.

Don't worry about it, we all make mistakes!
For a 7XL, you can go 5s or 6s. 5s would feel as powerful (if not more) than 18cells, but its speed might be slightly lower. 6s would give insane power, but would be a little harder on the controller since it has to handle so much voltage, and more amps (because the voltage is higher). So, I think you would be happy with 5s. And 8400mah would be fine, as long as the motor isn't overgeared so that it gets hot.

Thanks for correcting yourself, Ryan. I thought I was going crazy or something(I knew you had a 7xl, but couldn't figure out why you were aying 9L). 5S Lipo would be the way to go IMO. It will have more torque than the 18 cells you run, less weight and close top end speed (maybe slightly lower, but not noticeable, really). A pair of 5s 4400 mah packs would get you a nice runtime and nice power.