I run a #36 jet which puts m in the 40 hp range. My 2lb'er seems to last quite a long time. I never tried to put a stopwatch to it. If you're going to do this, go with the larger bottle. Make the fun last as long as it can!
Don't worry about a richer nitrous map being in your pc all the time. You're only going to be on the juice between 7-11,000 rpm at wide open throttle anyways. I don't think you're going to feel a difference in the map when you're not on the juice either. It's not going to be that rich.
It's interesting. I had all these same questions and more when I thought about getting nitrous. Everthing seemed like a compromise. After the install, I couldn't be happier! Nothing like taking a really fast bike and making it NOTICEABLY FASTER! It's the best bang for the buck too!
BTW, my A/F is 12.2 - 12.3 when I spray.