SS's dyno measures drivetrain loss of deceleration-- what they're showing is flywheel hp.
Green line is drivetrain loss.
blue line is wheel horse power
light red is flywheel hp before headers (blue + green)
dark red is flywheel hp after headers
orange lines are torque before and after headers
275 (+20 hp) flywheel hp is where a factory tuned DISA/SS car should be. That should be the minimum stig has. Add to the the Evolve tune gains and performance intake gains, and you get where he actually is. My guess would be 285-290 flywheel hp.
285 flywheel hp with 17% drivetrain loss (typical) would put him at 237rwhp. So... that's what I would expect.