I went back and tested the ceiling with a 500fpm climb rate, and it was only about 1000ft lower. That difference would be barely visible on the chart. So to me it still apears that the ceiling difference between dcs and the flight manual is over 10 000 ft.
Temperature has such a significant and backwards affect in dcs that it could be the cause of much of the differences in altitude and top speed.
In various other simulators, I noticed that usually increasing weight increased top speed, because the angle of attack was negetive at such high airspeed (most wings produce lift at 0 AoA). Adding weight can help align the fusalage and wings with the airflow. This needs to be concidered when designing aircraft, selecting the wing, and its angle of incidence.
I did a quick test in DCS, and weight had no measurable affect on top speed. Top speed was mach 2.606 at ~1deg AoA, with both 0 fuel and full internal fuel. I was unable to produce a negetive AoA at 1g, even after diving to the ground and exceeding Vne. Going inverted at top speed resulted in about -1deg AoA. These tests suggest the F-15 wing as modeled produces near 0 lift at 0 AoA.