Is there any advantage to adding MSD to my 454 mag mpi procharged ??

yes there is, you will get infinitely better fuel mileage.

now the reason is you will be getting towed back to the dock when their crappy products break. seriously find something better.

multi spark is turned off at 3000 rpm i think or it used to be and yes the blue boxes do fail

An ignition box will give a better spark and help to keep your plugs from fouling

Ive consistently seen 10+ hp gain when running a good ignition box on the dyno vs none in back to back testing. had alot of problems with ignition boxes on a supercharged 540 I had in a boat. because it was a mefi with factory wiring I could bypass the ign box in a 2 minutes, never saw any speed or top end rpm difference when on water with or without. I early days of running supercharger before I started doing in boat EFI tuning , my msd seemed to help keep plugs from fouling as easy, once I dialed tune in, that was a NON tissue

Take a look at Daytona Sensors Marine CD-1. Way more features and better made. It comes with a wiring diagram to tie into the Merc ignition. I have a big cam and high compression, the idle control is essential especially for docking. It also has a feature to retard timing on an input. I run pump 93 octane, but if I have to get 90 octane marina gas, I can flip a switch and retard timing. You can also tie in a MAP sensor for blower motors to change the timing based on boost. I paid about $450 for the box and coil.