James: I agree with almost all of what you say, except for a fine point:
Bitcoin isn't really efficient for FX or any transactions. Very little of the cost of transactions--domestic or foreign---are actually sending the funds. They are in fraud prevention, customer service, legal compliance, and risk management (including exchange risk).

Not only does citing a blockchain transaction fee neglect all of these services, but also leaves out the fact that miners are paid by newly minted Bitcoin (the mining reward). You can see on https://www.blockchain.com/charts/cost-per-transaction that the mining cost per transaction rarely is below $30 and we haven't yet priced the rest of the process.