Guys, I sure don't want to be held to this, but I sort of a gree with Dave's comments, about the flow of water out of the boat (on a big block 38), downhill from the riser right through the muffler. An aqua lift situation may well provide more protection, especially in a following sea or diagonal wave that would smack up against the exhaust port and want to force water into the motor. Take a look at my installation, you can see it's "almost flat" and I'll have to put a level on it to really see which way the water flows when she's sitting at the dock.
For my purposes on the inland waterways, the backwash issues due to waves never entered into the equation. I'm still using original iron mufflers, they're still working as intended, and I don't think the system is lifting any water (it appears to be pushing it sideways away from the riser during the time the motor is running. I'll have to check this real close next time I'm aboard, because I'm very interested in the true flow here, and I don't want any water backing up against the riser while the boat is sitting, as Dave suggested.
Regards,
Paul