When to inform ATC will largely depend on where you are - the sequencing at Heathrow being much more speed dependent than at smaller airfields. But if I'm putting together a sequence of four or five, telling me when you're on a base leg or with 15-20 miles to run is enough time for me to sort things out behind you. The big consideration with the 757 is vortex - so a slow final approach speed can erode that separation quicker than might be anticipated.
The only time I'm really amazed by the 757s slow final speed is when helicopters catch it up on the ILS!
