If performance is that critical, maybe you should write this section of code in C, and interface it with Blitz3D as a userlib. Because I don't think Blitz3D is capable of what you're trying to achieve.
But it's puzzling! What does blitz consider as a string? Can't a bank be considered a string? Without any data conversion? It doesn't need data conversion does it? Or maybe I need to trick blitz into thinkig the bank is a string...
hmmm...(!)