I agree with you guys. Well over $30K ($22K plus install plus additional parts and install) to get to a true 503 hp is a ton of money to pay for an extra ~90 HP. If you instead use that money to get a slightly used GT3, you get a car that performs better in most ways, AND holds its value much better (you won't recoup much of the cost of the 4.6L; GT3s hold their value quite well).
If the total price, including install, was <$20K an argument can be made.
Seems no matter how much they charge for the kit if they sell almost 0 then they will never re-coup their costs.
Driving sideways: It's not faster, but damn it's more fun!