Bikes of that quality were designed and spec'd to use a 150-200mm post.
Even so, you'd only have about 40mm of post in it unless you did the braze-ons as they did...not exactly much adjustment in it.
If it inserts past the top tube to the minimum insertion line on the post it's fine, there plenty of room for that and without seeing the way that boss is designed there's no way you can just assume those bolts will make it inside the seat tube in the first place. Beleive me it's adequate.... they wouldn't want the warranty issues. If you buy the right size frame to begin with you don;t need a lot of adjustment on a road bike post it never gets moved once it's set.
GT's Spoken here. Current projects: 92 GT Team member RTS-1, NOS 94 GT Edge steel viewforum.php?f=49