so I ran some more tests. Using the 1700ltr drop tanks was not the right benchmark. These tanks were not supposed to be used in combat, but only in long range ferry operations. The Argentines used them because the 500 ltr drop tanks designed for combat did not give them enough flight time over the islands. However, both the 500 and 1,700 DT were used in combat.
I reran the same tests with the 500ltr drop tanks in both SF2I and the mod, making sure the planes had the same weight. As soon as I take off, I use alt+n to warp to the IP at 35,000 feet.
test #1 - SF2I, initial weight 20,690 lbs, 68% throttle. The planes start at 275 knots and quickly accelerate to mach 0.96:
test #2: SF2K, initial weight 20,590 lbs, 68% throttle. The planes start at 279 knots, but are not able to maintain that speed and gradually slow down:
so there is an issue of more than just weight. The Mirage IIIE appears to have less thrust than the Mrage IIIC.
In RL, the Mirage IIIE was powered by the Atar 09c engine which was more powerful than the Atar 09b engine used in the Mirage IIIC, although the diffrence may have been as little as 6%.