No pds slot in an 840av means that the fastest stock Mac one of these works in will be a 33MHz machine (Q650, 800, 950). What would happen if you were to put it in an overclocked machine is another matter.

DayStar (I think it was) made these for Apple, and themselves produced a later, faster version (or two?) under their own name.

You are right about 68k software generally working faster on the 840av. However, the superior performance of the 601 in applying Photoshop filters and the like still made the upgrade attractive among the design set back when new.