I was thinking about this, but I am not sure if I feel save with spacers on a 420HP car.....
As long as you don't get carried away with spacer thickness I don't think you'll have any issues. You'll have to do some math to figure out what you need but I don't think it's very much.
I just picked up a set of eibach 25mm spacers as I am installing brembos and want to be able to use the factory 18x8 rims for when my car is in storage and when the weather is too cold for summer tires.
Are you sure they are 25mm? That's a lot.
There is a massive difference between old drum brakes and modern disc brakes, there's no comparison. The difference between Brembo and standard Mustang brakes means very little in comparison to the difference better tires make.
I'm pretty sure he was being sarcastic.