I remember hearing that 75 mph was a good general minimum speed for a spoiler to have any function at all, and for the spoiler to be creating useful downforce 100 mph seems to be the minimum. Having seen the z28 and transam OE spoilers I can honestly say that they are too small and at an insufficient angle to produce any useful downforce.
Back on topic, take the Sentra to a body shop and have them attach the spoiler and match the paint, there are too many ways to screw up bodywork if a person is new at it. Trust me, it may look OK when you first mount it, but after a year or so you will be wishing you had it professionally done.
Zenmervolt
EDIT: iamwiz, "porche"? I think you mean "Porsche", there's an "s" in there. Just making a friendly correction, no flames about me being the grammar police please.