Using the formula
(in US units)
This is just basic physics, accelerating a known weight over a known distance and using the ending velocity calculating the power it takes to achieve that velocity...
To make the math easy, you can use the calculator below...
http://www.ajdesigner.com/fl_horsepower_trap_speed/horsepower_trap_speed.php
and estimating the cars weight at 3350lbs (car at 3200lbs and driver at 150lbs)
Center Delete: 121.4mph = 467.8bhp
Dundon Race: 126.8mph = 533.0bhp
If the car and driver is 3200lbs
Center Delete: 121.4mph = 446.8
Dundon Race: 126.8mph = 509.2
If the car and driver is 3100lbs
Center Delete: 121.4mph = 432.9
Dundon Race: 126.8mph = 493.3
Estimating the weight at 3350 gets us pretty good alignment with stock numbers at the crank... (car is advertised at 469 SAE HP)
This was the same driver, same launch, same tire pressures, same track, same rear wing removed...
This also would include the effect of ram air which we know adds power, but is hard to reproduce on the dyno. Also changing the weight of the car has an obvious effect on the numbers, but the delta stays about the same. It also looks like a center delete may lose a bit of power, but would need more data.
This is more power than we record on the dyno, but it isn't a surprise as the decklid is designed to capture air and force feed it into the engine...
You're all free to go run the numbers or play with the calculator yourselves...
The increase was enough that the race tech guys pulled him out of line and searched the car for NOS... 😉