If you're wanting to know if Groovy could be the best language to use for every application in a bank... I don't think so.
Each language has its strengths and could be best suited for certain situations. In some cases maybe COBOL or even assembly would make the most sense.
I think Groovy makes development so quick that it would be worth, at the very least, to prototype most applications using Groovy. If the performance you see in the prototype is unacceptable, worry about it then. Even in a scenario where Groovy isn't optimal performance-wise, you're able to get a functional product to the business folks so they can play with it and give valuable feedback early in the project.