I wouldn't say the backerboard actually has any "give". I mean, there are 54 screws in each full sheet, so they're definitely fastened down.
It's actually the gritty "sound" that bothers me. I can certainly run a lot more screws into it, that's no problem at all with the gun. Would only take a matter of seconds. But I'm afraid the "gritty-ness" will still be there.
Bottom line is, if it has to come up, I'd rather do it now, rather than once it's covered with slate and grout.
Randy