For the needs you describe (ledger of inter-bank transfers) we already have established solutions (all the large RTGS, real time gross settlement systems around the world) and it's not clear what advantages, if any, Bitcoin has over them. Cryptocurrencies have certain advantages in certain (niche?) use cases of end-users; however, if anyone is building a Visa-like product on top of something, it won't be any advantage for your users if it's on top of Bitcoin and you might as well go with the other RTGS systems that (among other things) make you less exposed to various risks.
