As a technician, I've seen plenty of drilled rotors with heat cracks @ the drilled holes. It's not a certainty that every drilled rotor will develop these cracks, but it's more likely than with a non-drilled rotor. I haven't seen any increase in warped rotors when they're drilled &/or slotted.
I've used the Powerslot rotors on previous Mustangs with success; in fact, they kept the pads I was using (Performance Friction) on my '90 LX quieter than the exact same pad on my '91 LX but with stock rotors.
The stock Cobra brake system is very good, & OE replacements are just fine. Every penny spent improving their performance, though, is always a good use of money in my book.
I'd go with a good quality pad (Hawk, PFC, etc) & the Brembo replacement rotors. If you want the looks from drilled &/or slotted rotors, go for it. Just don't buy some no-name Chinese import, & you should be fine.