Yes the price is outrageous, but 500hp, 400lb-ft with full warranty is pretty sick! The "factory approved" aspect is why Dinan is able to charge so much.
A modded car will always be a better value; with $100k I could buy and mod an E92 M3 that was much faster than a $120k M3 GTS. Although I would never buy the 4.6 stroker, if money or even value wasn't an issue, it would still be a sick M3!!