You should check the manuf. recommendation regarding is it ok to shim that much. According to HS, their two largest tubs should not be shimmed at all - flat/level/solid surface only.
Makes sense to me that the answer (is shimming this much ok) would depend on dimension of tub, thickness of shell, flex/rigidity of frame, etc. In other words, might work for one model but cause damage to another. Just a guess but I'd say better do some research and proceed with caution.