I've never heard of any photographers going to High Park for BIF shots, but I've spent quite a bit of time at Cranberry Marsh hawk watch, and it's quite good there. Sharpies, Kestrels and Merlins will fly very low across the marsh, often come at your head on, some of the eagles will come low across the marsh as well.
Many of the Red-Tails, Harriers, Roughies will hang around Hall Road since it's a good spot to grab a bite.
About 400mm, ie a 300/4 + 1.4xTC will be preferred. The platform can be a bit crowded on good flight days, so a 500/4 is a bit too cumbersome.