Through Transom Exhaust?

[FLexpat]

I have had it both ways and - like everything else - there are advantages and disadvantages to each. My 20 Bertram had a Volvo 290 w through hull just below the waterline and my 23 Sceptre had a Y tube through the drive (although it is all out during current rebuild). With the Bertram I needed it out through the transom since there was so little room (and I overfilled it); the well was designed for a 165HP I6 and I had a 290HP 5.7 V8 in it.

Through drive Pros:
quieter - especially for the neighbors and those in the boat following you
Factory flappers inside tubes usually work pretty well

Through Drive Cons:
Hard to tell if flappers inside the Y tube are not working well
hard to tell if you have corrosion issues on the Y tube in your bilges (unless you use a borescope or pull your engine periodically)
Another boot to maintain and that can be a PITA

Assuming the through transom goes out just under waterline - you probably don't want it above the waterline...
Through transom Pros:
Easy and cheap to maintain flappers since they are outside the hull
'Probably' easier to see all the fittings inside to inspect
Easy to tell which side of the engine you are having problems with (e.g. leaking valve stem seal)
Easier to hear when you are having a misfire - I heard it starting to slightly misfire on the way to Bahamas once and was able to get back to St. Lucie inlet just as the engine totaled itself
Less of potential leakpath to sink your boat (debatable)
Through hull cons:
Noiser in the boat and much noisier for folks behind you when you are on plane
Slightly noisier at low displacement speeds - noisiest at 'mush' speeds

Myths (my opinion)
I doubt there is enough of a performance difference to measure either way on a 5.7 but folks love to argue this
I doubt you raise the drive temp by running it through the drive unless something else is wrong - that raw water pump should be low pressure, high volume so lots of water does lots of cooling

I wrestled with the idea of putting it through the transom of the Sceptre now but am currently leaning towards using a Y pipe through the drive since I will probably pull the motor every couple of years for inspection and more importantly my wife does not like it loud at all - (marginally reformed ragbagger). I do like being able to hear the engine though...