Typically, there are two main ways to look at drivetrain loss:
1. Fixed Loss = This basically means that no matter how much power is being made at the crank, the drivetrain will always absorb the same amount of power. In the WRXs case, it’s said to be about 62 hp.
2. Percentage Loss = This assumes that the ratio of crank horsepower vs. wheel horsepower remains unchanged. In the WRXs case, it’s said to be somewhere around 20-25%.
So, given that your car makes an additional 65 wheel horsepower over the stock 160 (another estimate), let’s say you make somewhere around 225 wheel horsepower.
By method 1, 225 whp + 62 hp = 287 chp.
By method 2, assuming a 25% loss, you might have 300 chp. Assuming you have a 20% loss, you might have 281 chp. Averaging the two, you might have 290 chp.
Based on all of these rough estimates, my best guess is that you have roughly somewhere just shy of 300 crank horsepower.