It looks like we're referring to the same source: "USAAF Jabos..." (William Wolf, Schiffer). It's interesting that HVAR was actually used in the ETO before the M-8/M-10 was. The M-8 rockets were first used in August 1944, as you state. In July 1944 the American HVAR was first used in the ETO.
On July 17, 1944 several P-47Ds of the 513th Squadron of the 406th FG attacked the marshalling yards at Nevers, France with HVARs. A flak tower was destroyed first, then the planes fired 48 rockets, 38 of them hitting 25 locomotives. 3 repair shops and a roadhouse were damaged. That's pretty impressive for the 1st use.
On July 26 the first American rocket attacks against tanks was made with 4 tanks destroyed by HVAR rockets.
On the 27th, the German aerodrome at Coulommiers was attacked with HVARs resulting in moderate damage (1 large hangar, 4 small hangars, a fuel dump and 1 Me-110 were hit).
On August 13 the 513th attacked, with HVARs, 4 heavy tanks (Mark V Panthers) and 1 light tank moving along a road near Marignym, France and destroyed all five.
Despite the effectiveness of the HVAR (and the M-8/M-10), rockets were never widely used in the ETO by USAAF FGs. During the entire war in the ETO, the 9th AF expended just 13,959 rockets. Compare that to the PTO, where Navy and Marine aircraft alone, from August 1943 to August 1945, expended 210,056 rockets.
Regards,
SkyChimp