In my personal opinion 250 kts is pushing it as far as See And Avoid goes - in some recent flights in an Aztec over southern England we had three near misses (two involved helicopters). Even with three pairs of eyes in the cockpit we saw the oncoming traffic too late to take action, and this was at 140 kts. Pure chance every time. At 250 kts in a two-man cockpit during climb or descent with heads down configuring for landing or further climb etc., I do not believe See And Avoid is a valid concept in collision avoidance. Have you ever actually been in a situation in a jet where traffic was brought to their attention with a Mark 1 Eyeball AND in sufficient time to avoid a hazardous situation?
On the other hand: "Better to be a quarter of an hour late in this world than a quarter of a century early into the next."