Here's my take on your question. 5200 is likely to work, and for some reason many folks like to use it. The mfgr recommends it because, to him, there is no downside (it won't leak, and the customer has some on hand). Unlike deck fittings, chainplates, etc, in the case of through hull fittings, there is likely to be zero motion (ok, that's not strong enough -- THE FITTING WILL NOT MOVE), and so the 5200 won't be stressed, tempted to crack, or have any chance of failing.
The downside is years from now, when it needs to be replaced. With other sealants, you can remove the fitting, perhaps with some work. With 5200, you may well end up damaging the hull while removing the fitting! And when the next owner is fighting with the problem, he isn't likely to curse the fitting mfgr -- he'll only be cursing the PO's choice of sealants.
As an aside, I had to replace my engine through hull on my '79 Sabre 34. The bedding was grey, soft, sticky, putty-like, and in all respects resembled butyl. Sabre says they never used butyl below the water line, but I'm convinced it was butyl and remained water tight for nearly 30 years.
Harry
