Thanks for your replies Tony and John. I don't really think that there'll be a worthwhile difference in top speeds with the engines in identical jets. Probably more important is the relative turbine acceleration and deceleration figures, and the amount of residual thrust affecting the landing approach onto a short runway in a tree-bounded sheep field. My 44s are a bit slow on both accel and decel so I set up a flight idle of 100k rpm which helps during manoeuvres, but is far too high for landings. So I program the normal 55k rpm idle to be active when the gear is extended, but the resultant slow "slam" time can make an overshoot and go-around something to be avoided.

The Wren pipes are claimed to slightly increase available thrust due to their divergent first section, a feature other pipes don't seem to have.

Gordon