The numbers seem low because they are from a Mustang Dyno. If it were on a Dynojet, it would be 288 RWHP and 325 RW Torque. That translates to 339 hp at the crank and 383 ft. lbs. of torque at the crank.
Given the stock motor puts out 215 hp at the crank, I ended up gaining 124 hp at the crank (dynojet numbers) with the addition of the blower at 7 psi.
To give another comparison, the car was putting down 170 RWHP (on the Mustang Dyno) prior to the blower. Now it puts down 240 RWHP, for a gain of 70 hp or 41%. I am told that, without intercooling . . and with a fairly "safe" tune . . one can expect about 10 HP per pound of boost on a stock motor. My engine is stock and has 103,000 miles on it.
All of that being said . . dyno numbers are BS, whether its a Mustang Dyno or a Dynojet. Dynos can be wildly manipulated. I'm sure I could find a Dyno that says I'm putting down 375 HP at the wheels.
Another little comparison . . if you use a "horsepower calculator" (commonly found on the internet) and you plug in the weight of my car with me in it (3,750 lbs.) and my trap speed (99.24), it says I'm putting out 286 HP at the wheels and 338 HP at the crank.
The fact of the matter is, I'm running 305/45/18 Nitto Drag radials and they spin like the bujesus off the line. The issue is traction, not HP.
However, I just ordered a Snow Performance Methanol Water injection kit to cool my boost. With the cooler air temp, I'll be able to run more boost and more timing . . . so the "HP" will come up.
Bottom line is I don't really care too much about what the Dyno says. I know my car will run mid 13s the way it sits . . if I learn to drive the thing . . lol. I need to work a little more clutch into my launch and try not to bust loose as badly. With the Snow Kit on the car, I MAY be looking at 12s if I can get the car to hook.
Sorry for the long reply