Our Flying Manual for the 757/767 states that RETs are designed under ICAO guidelines for a maximum speed of 50kts and thus exit speed should never exceed 50kts in good conditions. Older RETs are not built to these guidlines and thus care should be exercised when exiting at higher than normal speeds.
Being told where to expect to vacate whilst still in the air can be useful, especially if a longer than normal roll-out is intended (this works well at EGLL when rolling for block 17 or block 10). In my experience the results are less good when a shorter than normal roll-out is intended. This may just be due to the taxiway geometry of the places I fly to though.