with a supercharger you dont always need to do some interals... they cant hurt but i have heard not always neccassary. AA tuning has put there supercharger on a stock motor with 150 thousand miles with no internals and it ran fine.
As for a turbo claiming 300hp on your motor, that is very possible, it is also possible for it to make 350 400 500hp etc "throwing" a turbo on your car doesnt just give you X amount of HP, the bigger the turbo, better tuning, reinforced internals etc can all help how much boost you run meaning more HP, even the higher octane gas will make you run more hp. You have to remember with turbos you wont always be running 300hp, When you go to a track you could say put boost to 15 psi and run 100 octane gas... but when you leave to drive home you will probably run 7-9 psi on regular gas, which will lower you hp reading... as we have said before turbos are pretty complex specially on cars that dont come turboed....
Are there exceptions? yes you see those honda civics driving around town with there HUGE front mount and a loud ass BOV? yeah they are running 4psi and my 325 can still walk on them, they spend maybe 3500 on a "kit"
the aa supercharger says it puts out about 322 hp (to the crank) and its only 5700 bucks... i would defintaly go with something along this lines whether it be aa or not. after i swap my m3 motor ill eventually look into F/I a turbo would be nice, but i think ill end up going with some sort of s/c and have over 300hp and be happy
EDIT just saw the rest of your question...
adding any kind of f/i technically isnt great for your engine... but the car will die one day anyways so who cares... also you can always get it rebuilt and built back up etc.. .Dado is getting his whole moto rebuilt right now.
As for tuning a supercharger yes you can do it, i think aa might send some stuff to make the s/c run right... when i go with a supercharger ill probably have some other mods to my car, and will want my air/fuel map pretty smooth, granted tuning is a SHIT load of money im going to spend the thousands of dollars to get my car tuned to all my mods/all the extra air for the S/C