system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] would finding a Fully intact and complete STC with all blueprints, mean victory for the imperium?
gpt
Unlikely.

It depends on too many factors, if they could keep it secret from the Chaos and other races, could they decipher it, could they use the templates given their current state of technology and understanding and most importantly if they have the time to use the technology they found.

If the Imperium were however, to find a STC and keep it secret.  Let say...  Mars?  Use it to develop it's technology and then feed it through to the Astartes and their armies then it'd give Humanity a huge boost.  However you've still got the Chaos Space-Marines (not to mention their Gods), the Eldar, the Necrons and let's not forget the Tyranids.  

The Tyranids and Necrons esp, a blaster welded by a SM could cut down dozens, hundreds of opponents.  However when your enemy has hundreds of thousands, millions, billions (?) of fighters then technology is only as good as the person who welds it.

In short, it would no doubt help the Imperium if they used it correctly.  But unless the STC templates have some heavy hitting weapons **and** the techniques for manufacturing them quickly then the Imperium would still have problems.  But if the Imperium got the STC, kept it secret and equipped their entire fleet/Astartes etc. with this lost tech, then the odds tilt favorably surely?