you will def. see big power gains @ stage 2. No worries there. It really depends on the dyno but most stock sti's dyno 240-250whp hence expect 290-300whp with stage 2.
Assuming you can drive well your basically going from a car that can reasonably do a 13.0 stock to the 12.5-12.7 range depending on driver/tune. Its a very nice increase in power for how little work and money.
Bottom line is that it is worth the money and you'll need both the AP and the TBE to do further mods down the road if you choose anyway.