Yo-Yo,
Are the HUD airspeed indications not in KCAS or the Mach readout in error? The Dash-1 numbers are based on instrument readings within the cockpit - specifically KCAS and Mach. The performance is based upon what's these instruments show. If the HUD airspeed indications are valid - showing appropriate KCAS and Mach, then these should be used for all airspeed benchmarks.
Using a computed TAS can cause the flight profile between the points to be in error - slightly slow or fast airspeed indications, which affect the slope.
Here's another chart of Time downrange vs Distance. This chart shows the slope of the climb. The slopes, as you've said do indeed match up fairly well, however, as you can see the slope isn't the whole picture.
If the LOMAC F-15 were performing similarly to the Dash-1, the altitudes that they arrive at should match HORIZONTALLY. That is, each aircraft should arrive at the same altitude with the same delta in time and distance. This would show a line similar to the Dash-1 slope, but skewed to the left/right a specific percentage.
It does not show this, as you can see from the chart. The altitudes don't match closely on the horizontal axis after 10,000ft. You can make the argument that low altitude performance of the LOMAC F-15 closely APPROXIMATES the real F-15, but above 10,000ft, you can see it diverges significantly (appears more than a 15% divergence in performance).