Hi all,

Exhaust speed depends on pressure ratio of the engine. Some numbers:

KJ66 and derivatives: (RAm750, P80, SJ1700, etc): Exhaust gas speed around 340m/s , around 765mph.
M90: 450m/s. around 980mph.

So, on a plane flying at 200mph, the exhaust speed difference is high enough for that the KJ66 powered doesn't loose too much push. I remember to have read that the propulsive efficiency is better if the exhaust speed and vehicle speed are closer, this is the reason why a turbofan is more efficient than a pure jet, so, looking from this perspective, the engine with lower exhaust speed win...

My opinion is that, at our speeds, thrust is thrust.

GAspar