I'm sure many can poke holes in the calcs below, but my calcs would indicate no effective down force.
A boat sail force was given in a reference to be 0.00256 x the square of the velocity in mph-squared. For a first run, I assumed the Spoiler to be vertical - not applying a downforce at all - rather applying a braking or full drag force. Then I assumed the spioler to be at a 45 degree angle.
I assumed the greatest downforce (or braking in first case) would occur in the 16' straight where top speeds occur. This will obviously be a higher force than in the curves, where down force is needed - but for the sake of the calc, I went with this. For a range of calcs, I assumed the cars traverse the 16' straight in 1/32, 1/16, 1/8, 1/4, 1/2, 1, and 2 seconds. The associated top averge speed to traverse the straight would thus range from 349 to 5.45 mph. I like to think the real traverse time is 1/4 to 1/2 seconds yielding 21.8 and 43.6 mph, respectively. It's squirrely beacuase average speed is not the top speed, but I did enough velocities that I think the top spessd is taken is encompassed.
I assumed a spoiler size of .25" High x 6" Wide - overly conservative but yielding 0.001042 ft2.
The drag forces in grams were calculated to be:
0.030249 - 5 mph
0.120997 - 10.mph
2.304321 - 44 mph (= 16' straight in 1/4 second)
12.099668 - 100 mph
48.398672 - 200 mph
108.897012 - 300 mph
193.594688 - 400 mph
302.491700 - 500 mph
435.588048 - 600 mph
For the second case using the sin of 45 deg and using the same velocities, the down force in grams would be:
0.021389 - 5 mph
0.085557 - 10 mph
1.629385 - 43.4 mph
8.555675 - 100 mph
34.222701 - 200 mph
77.001077 - 300
136.890804 -400
213.891881
308.004308
So if the 43.4 mph average speed is a close approximaton of the top straight speed, 1.63 grams of down force is applied in the straight at top speed. 1.6grams is 2% the weight of a 75 gram car and 1.55% of an 110 gram car. Again the oversized spoiler and the use of top speed in the straight exagerates the calulations of the downforce in the curves where it is needed.
Would you want to assume 100 mph? 8.56 grams downforce is over 10% of a a 75 gram car - maybe appreciable. But rememeber, this force on teh rear of the car might do more to lift the guide from the rail than applying proper downforce.
In summary, negligible force in my opinion.
That's my best try - throw darts if you like.