When you say R&R, where they rebuilt, new sleeves, etc? I replaced mine after about 2 1/2 years and 250 hours as they were pretty much swiss cheese near where the tail pipes attach. CMI and Jeff were very fair regarding their warranty. It appears that regardless of how regularly you flush your engine with fresh water (assuming some of your boating is in salt water) the headers will deteriorate quickly if you have a shore power cord plugged in for any length of time (meaning a few days while the boat is in the water). Apparently the headers, with salt water inside, and the shore power create a current that literally eats up the headers. It's a galvanic corrosion as opposed to salt water corrosion. Chasing after the leaks is near impossible.
Using Salt Rid or comparable when flushing your engines seems to slow the deterioration significantly.