Well that already explains it. Unboosted it is really controlled. You could even chop easily. But topspinning and hard looping is quite impossible or more like really weak.
So what did you do with your old GTX rubber that you gave up on? Are you just throwing it away?
After playing several games with the GTX, I can really feel myself losing a lot more points with this rubber. I can't get as much spin as I could with boosted H3, and the straight trajectory causes me to intuitively/subconciously brush on the ball more, trying to add more curve onto the ball. This results in a lot more whiffs as I swing over the top of the ball and miss completely.
At the same time, I don't want to throw it away and let it go to waste. That would be a shame to waste it.