I'm not sure how it would affect the horses, but being very local to Biggin Hill airport, my old riding school would often be buzzed by jets on the airshow days. A lot louder and a lot lower than any airliner would be over Newmarket, it didn't seem to bother the horses at all.
We are also right under the Heathrow flightpath, a stack being right over the Biggin Hill beacon and we regularly get a/c flying over in the holding pattern. Again it didn't seem to bother the horses at all.
As Warbler has already pointed out, Lingfield is directly under the final approach for Gatwick and many planes have flown over, at a height of around 2,500 feet. Doesn't seem to bother them at all.
Not quite sure where the estimate of 33 planes an hour has come from, but I'm going to bore everyone here! If a "heavy", eg transatlantic, 777 is following a "baby", Airbus A319, Boeing 737 etc, the heavy will have to be 3 nautical miles behind the baby. But if the baby is following the heavy, it will have to be 5 nautical miles behind, because of the greater amount of vortex kicked out by the heavy. Now imagine a heavy following a heavy, within 10 miles, there would be two a/c. But if a baby follows a baby, followed by a heavy, there are 3 within the 10 miles. So there could be quite a variation on the amount of a/c at any one time. The a/c will also be within the control zone, so their speeds will be siginificantly reduced, lessening any possible effects on the thoroughbreds.
Sorry if this bored anyone, but I'm learning it as possible career and it's also relevant.
Personally, I feel it's a case of people being affected more than the horses.