The horsepower expectations are directly related to the thickness of your wallet. I.E. How fast do you want to spend???
EFI doesn't produce any extra horsepower, it is just a more efficient and arguably more precise and tunable way to introduce fuel to an engine. Horsepower is directly determined by how much oxygen you can shove into an engine, and therefore how much fuel you can efficiently burn. Tell us about the combination. Compression ratio, induction (NA vs. Forced), cam profile, head flow, RPM range, etc. Then we could offer up some expected performance numbers more precise than somewhere between 200 and 600.