How does the mercury 600 hold up with the stage 4 kit added. Is it a decent package or something to stay away from?

Great package as long as engine healthy when upgrades added. Big midrange torque. For greatest longevity, prop up to keep rpms near stock ranges.

how would you know?

Buddy of mine had stage 4 installed when he had engines rebuilt. Once all the bugs were worked out, it runs strong. He put Hering cleavers on it, and it was a really fast boat. I think the bugs were not whipple related, as I don't think the ECU's had to be sent back.

I did the stage 3 at 80 hours.at 200hr I pulled heads and had edddie young rework heads and springs and did stage 4 upgrade.it makes a huge difference and sounds like a real motor instead of a house boat motor lol