You know, there's a sticky in the General Discussion section for a Supercharger thread.
Yes, I know that. I checked it out, but was looking for more info on turbochargers + if a Stang would be a better choice for power.
Why don't you read that thread, and do a search for any additional supercharger threads, and have your friend make up his mind.
A Mustang is a Mustang, and a Boxster is a Boxster.
The only thing these two cars have in common is that they have 4 wheels and tires.
Not true. They have bumpers, radiators, gears, headlights, tail lights, doors, windows, "hoods", brakes, rotors... Etc. 
If you haven't noticed as of yet, this is a Porsche Boxster forum, and no one here really cares how much horsepower you can get out of a Mustang when you throw oodles of money at it. Bragging about that fact isn't helping anyone care much about you, your Mustang, or your friend.
Wow dude, get that stick out of your butt. Whats the matter with you? Did I manage to insult you? Yeah, Mustangs make a lot more HP and TQ than Boxsters, but the Boxster make up for it in curb weight, "balanced weight" (50-50 weight distribution???), and most importantly handling. If you need to insult me to get your point across, I definitely need to rethink my definition of a Porsche driver. 
Here's what I recommend that you do after you are done searching the forum:
Go find a Porsche dealer, and dress up fairly nice (business casual, preferred).
Ask to sit in one of their Boxsters, and have your friend get a feel for the car.
He might not like the interior or the seating position in comparison to your Mustang.
If the dealer asks if your friend wants to take it for a spin, be polite, and accept. Screaming "Hell Yeah, fire this bad boy up!" isn't going to help much. When he goes for a test drive with the salesguy, have him drive easy, and be reasonable with the car. That's the best way to get another test drive with just him and you behind the wheel.
Yeah, except he's not looking for a NEW Porsche. Something 15K or less.
Once you guys have actually DRIVEN a Boxster, then you might be able to get past the paper spec wars that you are currently engaged in.
Paper spec war??? Hmm... I.e. time slips? What do you mean? 0-60 numbers are tests... One test is better than 1000 opinions. Not that acceleration is the only thing. Your handling is much better than ours, & you can take us in the turns (stock 
).
And I'm sorry, but there isn't ANYTHING that you can do to make a Mustang with a live rear axle handle like a Boxster. Its taller, heavier, longer, crappier suspension design, crappier brake design, low revving engines, and just will never, ever feel the same as a Boxster.
REALLY!? Are you some sort of car expert here??? I guarantee with an UNLIMITED budget you can make a Mustang handle better than a Porsche. The cost would be huge, but nothings impossible.
Hell, if you guys lived near Colorado, I would take your friend for a spin in my '98 to show him what its all about. We have some fantastic roads that when experienced in a great car, will melt your brain, plain and simple. I'll even let you and your Mustang tag along, to see how it compares on a great road.
Porsche --> Luxury sport ~ Mustang --> Muscle sport. Your car better damn well outperform a Mustang. 
BC.