I know the workaround:
if you take a prepar3d jet sample you will find this:
Which is the chapter dedicated about increase thrust with afterburner. the problem is, this has not been included or implemented in msfs2020, neither is included in engine.cfg
The workaround is tweak n1 mach to thrust table:
if we take for example eurofighter typhoon which has static thrust of 13500 and 20000 with afterburner and you divide between them you have more or less 1,48148, that you include at the end in the last row of the table. So you will reach the 20000lbs thrust in game at full throttle teorethically. This isn’t how AB work in reality, but is somehow a fake way to do it. Of, course there are other variables like drag and such involved, but this is the way now to emulate.
P.S.
Regarding your question, no, I have never flown an F-15, but what they come to sell us here is far from looking like an F-15. It looks more like a stock msfs2020 tbm930 vitaminized