those headers and tailpipes in your pic are those gil headers with custom made tailpipes. i am thinking of trying to find a pair of those headers but instead of going through the transom going over and using a pair of mufflers at the tips. have you ever seen that.
they're "revolution marine" manifolds with EMI risers (who later on bought molds from RM). ive seen a few boats that go over transom with their risers, only problem is you'd have to angle them down, or run them dry or you'd risk sucking water back into your exhaust. trust me, they're plenty loud as-is and i have the option of running a 4" slide in muffler if i want it to be quiet.
if you are looking into center rise exhaust go peek over on offshoreonly.com to find a set, you wont find much around here or RDP for center rise. also i still have some 316 4" stainless pipe. 2x 4' long pieces perfect for exhaust. =)