It would make Malleable too strong if the Rapid Synthesis success rate was bumped anything north of 75%.
If Rapid Synthesis does "2 units of Progress" normally at a 50% rate - it is worth 1 unit
Rapid Synthesis with Centered (75% rate) is worth 1.5 units
Rapid Synthesis with Malleable (assumed 75% rate) would do 50% more, which is 2.25 units
I think they'd have to consider scaling it back. Imagine if it was stacked with Veneration and Muscle Memory? The synth's progress would be half done 75% of the time if one of these landed. As opposed to just 50% under current mechanics.