i just swaped in a new stock 2.5L STi shortblock into my wrx how many miles should put on before tunning.

Conventional wisdom says at least 1K miles. Better yet, wait until the leakdown numbers get really good and the oil consumption stabilizes.

http://www.cobbforums.com/forums/images/How%20To%20Break-in%20Your%20Newly%20Built%20Engine%20v1.02.pdf

shows some pretty good tips regarding breaking in a rebuilt engine

I stayed off boost 100% for first 100 miles. I had to drive around my town's residential street late at night to do it. I ran WG spring pressure (about 7psi) for the next 500 and drove it conservatively. After that I turned up the boost to 14 for another 500.

I came from the land of DSMs where you would test out the new block on the dyno :lol:. I think there is a good middle ground with break in. It is a turbo engine that will see boost down the road. IMHO, it was meant to take boost, so why break it in w/out boost? It should see some boost to get the rings to seat properly. Just be reasonable.