I think the main issue with manifolds getting eaten out was lack of maintenance (changing coolant and flushing engines out etc). I noticed with the G63B manifold I sent off to get repaired that there are a number of fins that act like baffles in the plenum, and the return port that connects into the thermostat housing is really restrictive (you might be able to pass a HB pencil through it - barely). The plenum base is welded in and this is what seems to corrode as junk from the block settles in there and can't get out. I ground out the welded cover and supplied my own replacement cover for the shop to weld in for me. Among other mods I've made I ran a big ass drill bit through the coolant return port to improve flow (think it was an 8mm, but I did it after the cover was ground out as not to leave any metal chunks from drilling in there floating around)