That and the R1Slipper Clutch really amped up the track perf of a street bike.

I wouldn't think it would be something that makes a huge difference in a steady state rpm engine system.

"Rated at 400hp, the 409 MPI engine is designed to replace non-catalyst 6.1L, 7.4L, 8.1L or 8.2L sterndrive engines in 21- to 31-foot single-engine boats and 28- to 38-foot twin-engine boats built before 2010 (prior to 2008 in California)."

Sounds like they are going after the broad market of 7.4L/454 Mag/502 MPI and 496 Mag/HO boats that are out there. At 400hp, they are close to the middle of the HP range for the Merc black big blocks. I wonder how much torque the 409 MPI's have?

See below. Lots of engines in that range of HP, some with DI.

I was wondering the same thing about the valves, had one engine builder saying GM was having issues but other then him never heard it from anyone else.

I would guess that number is very low. We have tried to sell them to a few customers and it never makes sense when it comes to the money. Don’t quote me as it’s been a while since I priced one but I think Msrp on the 409 Mpi is 33-35k

Would be good in my Donzi 22 Classic but to expensive for what it is.

The MAIN issue in automotive DI engines is that the "oily mist" from the PCV system is routed back thru the intake, where it "lands" on the hot intake valves and builds up a thick hardened coating. A marine engine would do the same thing, unless "somebody" rerouted the PCV vapors "elsewhere". Functionally, routing the crankcase vapors to a draft tube installed in dry exhaust would work. It would not meet current marine emissions regulations, but it would work to prevent intake buildup if you get my drift.

I've messed with several V6 and I4 AUDIs with direct injection, and a modification of the PCV system that runs the "final stage" of the crankcase vapors thru a catch can after the factory baffled PCV chamber and before the reintroduction into the intake, does dramatically reduce (but not eliminate) intake buildup.
AUDI themselves struggled with this problem and added intake port injectors to help WASH the intake valves during normal operation. THIS works.
I am not completely sure why a DI marine motor would be a "plus" in any category aside from emissions while warming up and a slight bump in midrange torque. Traditional port injection works as well as any other method, aside from partial atomization in a cold engine. Once up to temp, port injection solves more problems than it creates.
We have to remember that a boat at low cruise (let's say 2600 rpm) is requiring about 1/4 of its max HP to maintain that speed (about 100 HP) while a car at 60mph only requires around 15 HP to maintain on flat ground. A gasoline motor takes about 1 gallon per hour per horsepower. The boat above takes 10 gal/hr to cruise at 2600. The car mentioned takes 1.5 gal/hr on flat ground at 50-60mph.

The high pressure pulsed DIRECT INJECTION does a fantastic job of atomizing fuel in those tiny doses necessary for best cruise economy. The boat, however, is operating in a zone where THAT benefit is moot, and a 60psi fan spray from a port injector is ALMOST as efficient. So why invite the downside?

Agreed, MPI works well, nearly as efficient without the issues.

I think the simple reason why some of the marine motors are direct injection like the ones I posted is that they are based on the automotive engine and not designed for marine from the ground up.

Only Outboards are normally clean sheet marine designs, at least in the small boats most of us own.