I've had custom Spanish, English and French Galleons imported from
Pirates for a long time in the Wolfshanze Mod... to no ill-effect to gameplay from my experience (and nobody else has complained about them either). I'm expanding it further since I've never had problems before to other European powers (basically all northern/central/eastern naval powers... Mediterranean powers will still get Spanish-style Galleons).
The only time the Wolfshanze Mod tends to crawl, is typical of all custom mods... late-game on larger maps with a weaker/slower system. I've never heard anyone complain the game was crawling in the age of Galleons!
If every model in the game had a high polycount, especially well-used ones like infantry and/or cavalry types... then you'd probably take a hit where you can little afford it... but I think a few high-poly galleons won't kill anything... there's usually not that many floating around, and I'm not using the model with all 34+ civs... only a handful of European powers... and I've already tested it in-game with three European powers to no-ill.
Slapping a national emblem on an otherwise Spanish Galleon, just makes the ship look like a prize of war, rather then a domestically built ship. Not a problem you or I created surely... just the way it is. I don't think it's a cardinal sin to use the model in-game... let Geo poo-poo on the idea all he wants... his love-child, the
excellent citystyle sets are used by every nation for every age all over the map and definitely need to stay low-poly for obvious reasons... a few galleons, for a few years, for a few civs that are high poly won't kill anyone, and my testing backs that up.