It didn't seem to have any effect.
In any event I went ahead and uploaded the aircraft with contrails set for true airspeeds between 170 m/s and 200 m/s. Since we can't trigger the contrails based on altitude this is a close as I could get to restricting them to where they belong since it's generally difficult to reach these high speeds until you reach higher altitude without going into a power dive. Given a fullpower setting and a modest climb angle (5-10 deg), you get to 170 m/s true airspeed just about the time you reach 26,000 ft. To add a bit more visual variability, the contrails also get progressively thicker as your speed continues to increase from 170 m/s to 185 m/s.
P-38J-15-LO SN 43-28430, Assembled at Lockheed's plant in Burbank, California.
55th Fighter Squadron 20th Fighter Group, 8th Air Force
Based at Kingscliffe, Northants.
30 escort missions, 9 fighter sweeps, and two bombing raids were recorded for this P-38, with one enemy aircraft, two locomotives, and one torpedo boat destroyed.