[gumby79]

You said track days ... what form? Makes a difference on what is good V bad. Like everyone has stated test test then test some more. Tuft tests to determine if it is a spoiler or a factory aero mod. If your into drag racing then converting to a Bonneville spoiler could be an option for down force and aero+. I saw many last time to the drags in Utah.
If road corse then any down force at any cost may be worth the inproved times look at time attack cars they use this principle.
Aerodynamic visualisation fluid( clay disolved in water) will also show the points were the air separation is happening and then were reattachment occurs... like how after a snow storm your car is covered in road spray that almost always has the same patterns .
Aba on a set corce preferably 3gal or more to make the filling discrepancies (.001-.01) a smaller % value (makes for less noise in your results). If posable data log through a Obd2 dongle device. Useful at the track too.