it is, but its also a huge source of black steel. And possibly more minerals
gryphonia is full of metals and minerals. We're net exporters for them, right now, and I have hopes for recycling tech to become much better than what we have in RL, simply because that sounds a lot like something magic would be better suited than tech (some kind of runic array to dismantle something in its basic materials basically... but I'm digressing. Just a though for the far future).
we really only want the orichalcum mines in stormlands, and even that does not REALLY require us to conquer it.
If we can somehow make peace with the yeti (we don't KNOW what their society is like, EXCEPT that at least one group of them was made of slavers) we can just buy it, like we do with the yeti. Even if they didn't want to sell, it wouldn't be the end of the world, and we definitely have a lot we could offer for it.
The main advantage in Maretonia is from their population (magic users of the three pony kinds), even while the land has certainly a lot of value. The Stormlands are so much WORSE than it... there's no comparison, really.
While I would like to build a railway across Yakyakistan, the only other way for us to get there quickly would be by either airships or boats... Both options may be worth exploring in the future, especially if keeping a trade corridor with Equestria becomes a priority. Either way, Yakyakistan I can't imagine wants Griffon Armies marching about their territory no matter how friendly we may be.
rails for the yaks are definitely an option, but for the orichalcum mines... either a smaller railway to the nearest coast, and the establishment of a port there, or even just an airship landing port, might actually be cheaper. It depends a lot on how things end up going with the yeti really.