SM105K
Streetlight Grand Prix Champ/ IG @fafomotorsports
You have to remember that is 365 crank hp. Most SHO's on a stock tune on 93 octane put about 290 to 300 All Wheel Horse Power to the tire.Just my opinion but I don't think you're going to crest 400 horse on 87 without some internal upgrades. The advertised 365 horse these cars came with was achieved on 93. The ecm cuts back timing according to the fuel it senses and horsepower goes with it. Wouldn't be surprised if these things are closer to the 350 355 mark on 87. Meaning you have to pick up 50 horse with an air filter and tune, I just don't see it, I don't see it even on 93.
Maybe with some exhaust work to really reduce restrictions, a proper true COLD air intake, a really good tune and run it on 93 you might get close.
I'm sure the experience on this site will have some direction for you but I think the real question is your budget. If you're dead set against running 93 because of cost then the modifications needed to reach 400 on 87 would be way more expensive and invasive considering the car recommends 93 in stock form.
My advice, if you dont want the 93 obligation is to leave it stock and enjoy the stereo, they're quick as is and decently reliable as long as you keep up with maintenance both routine and preventative.
If he is looking for a 400 crank hp tune, then almost any 91/93 octane tune from a reputable (keyword being "reputable") tuner will do that. He would need the tuning device, tune, 3 bar map sensor, plugs, and a drop in K&N at the minimum. That will put 320/330 all wheel hp to the tire with good air.
It is incredibly difficult to break the 400 all wheel hp tune only. Ask me how I know. Some have done it, but they used either 93 octane, or e30+, and threw methanol on top of it while having their tuner get super aggressive on the tune.
Best bet, might consider selling the SHO and buying a 5.0 liter Coyote Mustang if he is dreaming of the 400 whp club.