I gained maybe 1mpg from my K&N FIPK, but the nice thing is you never have to buy another filter.
Another great mod - subframe connectors. Better handling and safer control over uneven pavement, plus the car holds together better (e.g. fewer squeaks and rattles) the more miles you put on it . . . thus, you won't wear this car out and need a new car anytime soon.
Good point about the K&N, if dad is so worried about wasting money on the car, the best thing you could do would be to buy an air filter you never have to pay for again right?
Also, anything that improves the quality, and longevity (like billet internals) doesn't seem like a waste of money to me. Just remind him if you had have spent some money in the first place the motor wouldn't have (wouldn't've?) blown up.
To keep on with the quality angle, I suggest giving your dad a copy of "Zen and the art of motorcycle maintenance." by Robert M. Pirsig, which is a book about quality, and explain to him you want to improve the quality of the car, so it costs less "in the long run". Not only may the book confuse him so much that he may think you're a genius for wanting to improve the quality of the car, he might actually understand the book, and agree with you.
You could also try the ignorant excuses. What loud exhaust? It's always sounded like that. That supercharger was always there dad!
Nope. The extended coverage was only 7years/70k miles first of all - that about excludes most 96/97s anyway. Then they replace the cracked NPI one with another NPI one. In the begining Ford used just the '98 all plastic version, but then later changes to an aluminum crossover version of the NPI one. So no upgrade if you do happen to be covered.