Ran mine for years without one. As I got faster and faster I added the small stock unit on my '70.
At about 140-145 mph it got a little loose once in awhile, it sure seemed to help.
It sure worked when I hit 159+ I can't imagine trying it without something???
Be careful if you take it off and try running fast!!!
If you really want to know if it's doing anything, it's pretty straight forward if you have shock sensors on the car. If you know your rear spring rate, you can log the amount of shock travel. Once you know the amount of travel & spring rate it's easy to calculate the pounds of downforce. Then when you make wing or wicker adjustments, it's easy to see how it affected the car. On our car aero is significant so it doesn't take much to see differences.
No data recorder here either.
Car runs 155-160mph so I will see what it does without it. Mine has a big rear spoiler that really sticks up in the air (I have a Firebird that has a TransAm style spoiler on it). I will find out what it does in a couple weeks.