I got mine at gamestop too, it is still a 4-day headstart. The beta is pretty much the full game with locked content. They'll patch it and it'll be the full game when you update on the 29th. At least thats how the guy at gamestop described it to me. As for the Subscription you should still be able to get any subscription you want, but as far as I know you have to buy it through the main STO site.
That's how I've seen other early-launch games work. A day or three before early-launch comes the mondo huge patch, bringing the installed client up to the release level. There's a character wipe and everybody goes nuts for the early-launch duration.
Come actual launch day, when everybody's getting their new copies of the game, it'll turn out the only thing you will need is the account key inside the box. (Your pre-order bonus keys will be in there, too.) The only purpose served by the install discs is for reinstallation.
Now, does this then mean the pre-order bonus stuff will or will not be available for the early-launch? I can't rule it out, Cryptic hasn't really given a clear answer... but it sounds to me that if people want their joined Trill or Constitution class ship or Borg BO, they may need to re-roll a character on Feb 2nd. (Hey, I'm prepared to be wrong about this, and actually hope I am. Be way better if, whatever level you are, you get the bonus thing as soon as you enter the code, whenever you enter the code.)
Correction: From the FAQ, which looks to have been updated since the last time I looked...
Q: If a player creates a character during headstart, then enters his retail key at launch (which opens up the exclusives), do pre-existing characters receive the exclusive items and costumes?