Josh,
In day VMC, SI's are flown mainly to save money. Anybody pushing the safety line is barking up the wrong tree. There are also plenty of times a downwind join will be cheaper than going out to 5nm final. Flying a circuit once in a while does wonders for one's currency.
Everybody has the same priority - so no, I do not agree that a SI should get it. The rules, unfortunately, are not totally clear, but if someone's on DW, then they have it over somebody outside 5nm final IMO.
When a highish performance aircraft in descending into a circuit, the estimate is exactly that, remember they are decelerating 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.
A crew of one of these types of AC, as Aeroscat has pointed out, should be able to get their estimate to within a minute or so. That's what they are paid for. An FMS does make life easier, but a join-the-dots box can still be used to give an accurate estimate.