I did it in multiplayer, and I only used one string that was very long. This was merely bank testing repeated, however, but I don't think they have any form of compression. I also restarted SC2 after each time I editted the bank file, just to make sure that editting it with SC2 open didn't mess anything up.
Did you preload it first and such? Basically all I did was have it preload, open the bank, read off the string in a debug msg, then save the bank.
Also, I don't think it got cut off, but I'll have to run tests again just to check.