T5's aint cheap but boy are they worth the money
The PC lamps get hot. The fixture has the lamps in a pretty tight space but the airflow around and through the fixture will dissipate a lot of heat. You are talking about putting a fixture that is designed to be out in the open in an enclosed area which prevents it from doing it's already tough job of venting the heat away from the ballasts and lamps.
If you use retrofits you now have the area inside your canopy for airflow. You should be able to find a kit that will be pretty much plug and play as far as the electrical goes.
Current makes a retrofit kit with 4x 65 watt lamps just like the orbit fixture.