Accurate plane performace?

I love FS2004 -- hell of a value for $50. I've recently been
downloading and playing with the slew of fan-designed aircraft. I
love speed, and so I've been gravitating to some of the supersonic
military aircraft -- Mig 29, F-18, F-16, F-15, F-14, SR-71, etc.

However, I've yet to find one such aircraft that performs in speed
and altitude the way that they're supposed to. The F-16, for example,
struggles to break mach 1.2 at 40,000 feet -- this is a plane capable
of Mach 2 at that altitude. The SR-71 can sustain Mach 3.3 for an
hour at 80,000 feet. The SR-71 I downloaded stalls out at around
70,000 feet -- at 60,000 feet, it's struggling along at 200 kts. (The
U-2 stalled out at 55,000 feet -- that got a good laugh from me.) All
of these planes can tear the MSFS world up at low altitudes -- I had
the SR-71 popping Mach 3 at 5,000 feet (which is also unrealistic; the
plane would fly apart at that speed at that altitude). But get them
to their cruise ceiling, and suddenly they perform subsonically.

Am I doing something wrong? Yes, gear is up, flaps are up, spoilers
are not deployed. Is MSFS not that good at rendering supersonic
flight? Or are the makers of these add-on planes not building them
correctly to interface with MSFS 2004? I've even tried going into the
..air file and doubling or tripling the engine thrust... doesn't seem
to help much at the higher altitudes. (Which are not really "high" to
these aircraft.) I'm puzzled. I'd love to fly a SR-71 mission 80,000
feet over Russia, but that's not going to happen at 100 kts.

-R