When a highish performance aircraft in descending into a circuit, the estimate is exactly that, remember they are deccelerating from say 250+ knots to 120, this will effect their estimate considerably, most crews will add a minute or two to the initial estimate, because this will be closer to actual.
If I can get it to within 30 seconds in a 'highish performace aircraft' them I'm pretty sure the two crew guys can do it too.
If you are in the circuit and wish to be difficult, say no, do not accomodate the arriving aircraft, I believe this to be bad airmanship and I am pretty certain it will bite you in the bum at some stage.
Yep, sure no worries, however, isn't it also bad airmanship to give bad eta's and then have other traffic (which would have been out of your way and on the ground) hanging around waiting for you...
j3