I think you'd have to be quite brave to base seperation on a filed TAS or requested level rather than actually setting a speed etc., so why any difference with type and hence wake category?
Yes, you would be brave. The difference with the wake category is that if you set it up on the assumption that the filed aircraft type is correct, then you'll possibly end up putting someone closer than is safe to do so.
For example, the RPL/FPL shows an Embraer 190 but the flight is actually operated by an Embraer 135 with no CHG message received. Seen similar circumstances enough times to know that there is a chance of it happening. The call of aircraft type as requested is a double check in a safety critical phase of flight. I'd rather 'waste' RT time with that than 'waste' my time filling in reports or worse.