That should be OK, it's what I used without a problem, on a 39 foot boat with about an 3 meter run from muffler to transom. (I confess I'm not sure what a gooseneck is in the exhaust system.) The muffler/waterlock should reduce the temperature to a safe level of heat.
Here is the spec sheet on 5200:
http://multimedia.3m.com/mws/media/1587 ... h-data.pdf
It appears that the safe working range tops out at 190 degrees, which is almost the boiling temperature of water. But it would have to heat up the bronze to that temperature.
There are also high temperature sealants for engine gaskets, of course, but I have no experience with using them on fiberglass.
TomW is probably your man.
Addendum: I just looked into the temperature rating of Shields marine exhaust hose, which is what I used and is common. It is rated to 200 degrees Fahrenheit continuous, peak 250, just a bit higher than 5200. But since you're at the cool end of the run and, like I say, the exhaust has to heat up the bronze to that temperature before it can affect the sealant, and because I have little doubt that Sheilds has built in a reasonable amount of headroom in their product, I'd still be comfortable with 5200.
Sorry for the long answer.
