Yes. But not enough to trust.
My friends (we have four Thrustline kits installed here, all on my students' Cubs) all report significant improvements in all areas. I will not enter the qualitative areas except to state that I personally do not detect any differences in controls, and to state that I may not be a good judge of qualitative handling.
It took me two years to learn how to strum a guitar and tap my foot at the same time. It took ten years in the Cub before I could fly with the ball centered without looking at it. I never will land on a big rock sandbar. So now you have me calibrated. I do have roughly 4000 hours in Cubs, and an engineering degree.
I will only have data on two aircraft, as the other two were converted without baseline data. If for some reason I do not get at least a 10% improvement, I will defer to data posted by someone who has gained such an advantage. I think a 10% improvement in takeoff distance would be way more than enough to justify the purchase of a Thrustline kit.
The kit itself is well done, and the manual that comes with it is superb! The only thing missing is the cotter keys. It may be one of the best STC packages I have seen.
I do drag Cubs down our paved runway for about a thousand feet with the tailwheel very close to the pavement (often rolling) and the main mounts as far up as I can get them. The only attempt with the Thrustline seemed to go very well, without a great deal of control input. Either I am getting better at it, or the aircraft was loaded better, or the Thrustline helped. I have no way of knowing which, and I will not guess. For me, this is a reasonably difficult maneuver solo - I had a co-pilot in the back, keeping the data.
I won't post more until next week. We need about twelve data points on each aircraft, on varying days, to match the baseline data. Takes time.