x2. If all you want to do is nip 14's, then staying N/A is fine, as a typical intake/rocker/downpipe/tune setup can get you there pretty easily. After that, though, a top swap (or turbo!) will get you more power for your dollar. To make decent power you need a good set of heads, milled as much as possible with thin head gaskets to really bump up the compression. Do all that and you might as well do a top swap while you have the heads off.
THAT said...the XP is an AWESOME choice for a naturally aspirated nitrous build. With its exhaust bias it is great for getting those gases out of the cylinders. With an XP and an open exhaust (read: headers), you'd be able to easily run a 100 shot, and maybe more. I've always kind of wanted to go this route since you get naturally aspirated daily drivability and gas mileage with the ability to turn on a bottle heater and go run 12's. It's something to think about.Not many guys choose the L36+nitrous route.