I'd have to say that there's good news and there's bad news on this one.
First, the bad news. He's probably right. Especially if you go from a dead stop. And even more right if he has a dual clutch auto with launch control. Although this particular launch control has been known to often be uncontrollable. You don't want to be beside him in a standing start. If he loses control and wipes both of you out, you will surely be blamed because of the poor opinion the public typically has about mustangs and their drivers. There is a certain reputation that is probably not your fault.
But there's good news, too. It's actually lucky he's likely to win. You think he's your friend but you forget that ALL BMW OWNERS are crazy and a bit weird. (I should know, my wife drives one...) So, if you beat his backside he will only pretend that it's okay but will resent you forever and one day will turn on you. I know it sucks, but DON'T BEAT THE BOSS.
If you want to have a bit of fun and keep it close, here's a plan. You will want to start from a roll. Our cars are not great from a standing start on the street, especially with the 3.73's. So, find which gear will let you punch it with no wheel spin. Let's guess it's second gear (not sure if you have auto or manual). You will set it up so you are cruising in this gear at exactly 4250 rpm (your peak torque rpm). You do the three horn honks and make sure you hit it, if anything, a bit early. NOT late at all. A tenth of a second is almost indistinguishable to the eye but makes for several car lengths at the end for two identical cars. Don't let him get any jump. If the coward does, just don't bite. Slow down and restart until you are at worst even or better yet accidentally get a slight advantage.
This should be really close. Close enough that you win when you point out how much he spent compared to what you forked out.
Now, if you really don't need the job and can do without his friendship, it's time to supercharge. You will still be able to laugh about how much less you spent as he tells everyone how terribly you thrashed him. If the company owner has any sense at all you will soon be your boss's boss. Owners typically likes someone who can get results while still being frugal.
Good luck to you and all the best in this undertaking.