1. yes. the later models had catastrophic constipators in the y pipe. but a replacement y or midpipe will eliminate those. hooray!
2. yes, yes. but make sure you don't put a hershey bar in there. they still get warm. make sure your bags have some type of heat barrier.
3. Satos are amazing quality. Akro's have an amazing reputation for making power. w/ a slip on, you will not gain much. the 2br stash pipe isn't very large, but it more than quadruples the available space that a 919 has to carry things. they aren't bad. single exhaust is fine, really, depends on what you like. same for brand of slip on.
4. you can ride your bicycle and offend someone. chose what YOU want, and ride how you want your neighbors to ride... you should be good
5. warranty? i have never used it for anything from Honda, so i don't know...
6. might as well start modding now! then, the next upgrade is that much closer!
7. K&N makes one, the gains are negligible. really, the cost of the filter+cleaner+oil... stock ones are cheap. and they flow a lot.
8. i don't like straight pipes. so avoid those. unless that's what you want. there is a recent thread about exhaust companies on the 'recent threads' right now. you can look there. the possobilities are endless!