Okay, lots of questions to answer.
First, reread this. (since you seem to have a decent grasp on things already, I'm assuming you've read it)
http://www.grandprixforums.net/showthread.php?t=67580
Firstly, you don't have to pull the motor to change the cam (I literally swapped cams 2 days ago, took about 10hours from removing the first bolt, to having it done and running)
You don't have to change thermostats, but most people do anyways for the 180* Tstat.
300HP is possible without going into the motor, but you will need an intercooler for that.
Don't give into the stage kit stuff. They usually contain a bunch of stuff you don't need ($200+ intake)
Best deal is to find used stuff on the forums from someone demodding.
The stock trans is decently strong as long as you don't beat this piss out of it, and you take care of it (Trans flush, new filter, etc) There are a couple people putting down good power with s/c or turbo setups with a stock trans.
First thing I would do is make sure all the maintenance is up to date (including the trans)
Then, a set of headers, and a home-made intake. (Just get a 4" cone filter and either put it on the TB, or make your own intake piping)
After that, decide if you want to mod it further. If not, get a canned tune from one of the vendors (I prefer Intense, personally)
If you do decide to mod further, I suggest doing a cam swap, or 1.9 rockers. Then get a custom tune.
As far as gas mileage, my old ported Gen III, 3.4 pulley, headers, intake, canned tune got 22-23mpg city, and 30-32mpg HWY. And currently, my untuned cammed 3.8 pulley Gen V setup is about 19city, 29HWY.
