Marine Power

Go to www.marinepowerusa.com. (http://www.marinepowerusa.com.)
Pretty good site, their forum sees little action.
The 454Mag/Premium motor is definitely the one to get. It can make a good base for HP downstream. The std engine is cast everything and does not want to survive with substantial HP boosts.
If you can, check out their X-motors.
Stay away from the new 8.1 block motors. The ECM is not calibrated (yet) correctly for the fuel mapping demands of jet drives.
From what I understand, the 8.1 is fine in Mercruiser I/O's.

I'd say Marine Power has the 8.1 (496)motors dialed in pretty good now. I have 40 hours on a new 21 foot Ultra Stealth with no problems. This motor pures and is strong out of the hole. Pulls my 205 lb butt out of the water with zero hesitation unlike my old boat with a 454, that one had a tendancy to cavitate a bit. I was was hesitant at first because it's a new motor but so far I love it.

I went with a carb'd 502 for my new Caliber 1. Almost got the 496 STD, except its length ate up interior space. Very happy with my decision.
Spoke with an owner of a new Shockwave 21 Skier who has major complaints about his 496. Popping, cutting out, etc.
From what I've heard, the 496 Merc. I/O is really clean, but not a pump version. So far, Marine Power's website does not choose to answer questions about this topic.

I have a new Shockwave 21 Skier with the STD 496 and Dominator Jet. I have only had it to Havasu 1 time and it ran really good (breaking in). Also, Shockwave just used it in the Hot Boat Magazine Jet testing at Havasu and they said it ran about 59 on the radar gun. That is what I expected so I am pretty happy with that number. I love the boat and they are great people to work with.