I just put a Rajapuri in 5-1-1. Seems happy. Bananas need a lot of water, so 5-1-1 is much better than gritty for bananas. I'm sure people put them in normal potting soil, banana stalks die after blooming and new stalks shoot up, so banana growers typically replace the mix every year when they separate the suckers from the dead banana stalk.

Last year I grew a "truly tiny" dwarf musa in 5-1-1 and it thrived pretty well. Started as a tiny plant (maybe 3 inches tall, with a few spindly leaves, looked half dead) in May, was about 3 ft wide and high, putting out leaves over 18" long each week by the end of August. I grew it as an annual, for fun, due to my zone and lack of space to overwinter it.

I gave it lots of fertilizer and water, had to repot three times between may and september, did not realize how fast it would grow.

Daniel