Here are two videos, one with 46% modifier at 0KFT and one with 100% modifier at 0KFT. Both have 100% at 20KFT.
Missile is set to 2000 kts powered and 1000 kts unpowered.
In the first video, you can see the launch and the missile not quite hitting the 2000 kts because we're at 10KFT in the video (cruise altitude is set to 40 aka 10KFT). Once you hit burnout, the missile slows down to the final speed and remains that speed constant until it starts to dive on the target. There is an instant loss of around 55 kts, likely the result of the maneuver to the dive. Then a slow increase in speed as it dives. But then we see another, even more drastic loss of speed (more maneuvers? lower altitude?), followed by an even more rapid acceleration before impact (maybe the dive?). The missile is set to go down to 1KFT at a cruise distance of 20 and 10KFT at a cruise distance of 78.
https://www.youtube.com/watch?v=gMl01JymQi8In the second video, you can see the missile going right to 2000 kts because I have the 100% modifier at 0KFT. Just as before, there is an initial speed loss - this time around 50 kts - and a gradual acceleration as it dives. Then the more drastic reduction (even more drastic than before actually) and the same increase in speed. I fired two missiles on this one instead of 1 so that we can watch it again but the performance is identical.
https://www.youtube.com/watch?v=8aYfJr-shCISo, at best, this is what I can ascertain.
1. Weapons go to max speed upon launch
2. Weapons go to final speed upon burnout
3. Weapons will gain speed in a dive
4. Weapons will lose speed in a maneuver
I surmise that if a weapon climbs it will lose speed if it is unpowered.
So, push comes to shove, the burnout speed remains a critical factor. Missiles won't lose speed just flying along but the burnout speed obviously is going to factor into the missile's speed upon diving, climbing, or maneuvering on the target. Ergo, if I were to just set it to 50% - as I did in this test case - it makes missiles (primarily we're talking AAMs here) ultimately easier to defeat. Missiles such as the AIM-120 have a very short burn (< 10 sec) with no sustainer. They're flying high and diving so they maintain their energy really well. In a few test scenarios in DCS, which seem to model the AIM-120C-5 very accurately - they're maintaining over 2000 kts at quite far distances. Actually, it appears on missiles like the AIM-120C-5 that energy isn't really a problem when it comes to range but rather battery life on the seeker. It's only around 85 - 90 sec. They still have a lot of speed but speed's useless if your seeker is dead. So ultimately, for a missile such as a long-range, "low drag" AAM like the R-77, like the AIM-120, like the Meteor, we would want much less of a penalty to the burnout speed than something say higher drag like the AIM-7, AA-6, or AA-10. Presumably the AIM-120A/B with the original Sparrow fins would fit into the "higher drag" category. In this regard maybe 80 - 85% of burnout speed is better for the lower drag and 67 - 75% for the higher drag. Then for dogfight missiles
now what because they typically have very short burns (< 5 sec) and bleed speed fast as they maneuver. Almost like they should be closer to 100% at burnout and then let the maneuvering kill the speed instead of the "final speed" factor.