a shift kit will not prolong the life of the engine.
It will increase the top speed opf the bole tho since you can have more than one gear to go through (like a transmission).
the bonus is that it gives you the best of both worlds...low gearing for good take off, and high gearing for top speed. It will allow you to cruise at 25-30 mph in a higher gear, which will make the engine run at lower rpms. That would be less strain on the engine, and higher mpg.
I personally would not buy a shift kit simply because of the cost. I can build 2 single speed motorized bikes for the cost of one shift kit.
boostbottles...some people say they work and some say they don't. a boost bottle can work as long as the engine is set up to make it work (this requires internal modifications to the ports). i have read about people just bolting a boost bottle on and then saying that they notice a difference in idle, the engine is easier to start, and it gave a little more performance. I bolted one on and the only difference i saw was that my wallet was $20.00 lighter. so, boost bottles are debateable.
an expansion chamber will definitly give you more power as will a better high flowing air filter.
also consider going to an ngk spark plug, and an automotive style spark plug wire with a rubber boot on the end.