How fast can you go with a Magnum? Who has the quickest setup?

Yes and No.

The bottle neck IMO starts with the intake manifold. Then, the throttle body. I could have it reversed. The computer Chyrsler uses isn't street user friendly. Or, in other words, you have to know a person to reflash it. The injectors have limitations. I do not know where that is. There rated at xx pounds. You have to know where xx pounds will show it's hp ceiling.
I don't know where you want to go with this build.

FWIW, A MP - M1-single plane just hit the classifieds for $275, used, glass beaded. That intake is best used at 4,000 plus RPM for the strip. Otherwise, hughesengines has a RPM with FI abilities due to the cast in FI bosses.

No aftermarket EFI experiance here.