re: What is the Popeyes secret spice?

They use a buttload of red pepper but it's a very specific one. It's bright red, fresh and standardized to a specific heat level(Scoville units). Most cayenne at stores is dull red and dried out.
The other secret is frying in a mix of veg oil and non-deodorized beef tallow.

Season with salt, white pepper, garlic, dry mustard and msg. For spice, order some 40,000 SHU cayenne powder off Amazon and when you think you've added enough, double that and let it set for at least 12 hours if it's whole pieces. A lot of the spicyness is going to cook off when it's fried.

For the oil, use the shortening in a yellow can at Walmart that say "animal fats" in the ingredients. It's as good as you can get without rendering your own tallow.

So yeah, the main secret is to use way more red pepper and use better quality red pepper.