1- Even if you look down at that height, you won't map enemies as well as you would at 10k. Keep in mind that your radar bear is a cone, when you point your cone down at a very high angle, you will «efficiently» cover only a small area, and at limited height. Foe exemple, try to scan from 0 to 20k feets at a scale of 40, then zoom out to 80. What do you see? from 0 to 0, wich pretty much meens that you can't cover 40 miles at such an angle.
2-the F-15 has an operational roof of ~65000 feets, so it is normal to be harder to stay high. The higher you go, the harder it'll be to keep your speed, near 20k Kms (around 70 000feets) you can't keep your height whitout loosing a LOT of speed.