Not yet. I purposely wanted to keep it not too aggressive to start. If you compare the spring resistance of the OEM valves vs the Flat6 with silver springs with your fingers, it is already a night and day difference.Great feedback. Do you wish you would have changed out to the Yellow springs?
When I say risk of flutter at >12PSI, it is probably like 1 in 20 hard pulls it might flutter.
On the flip side for comparison, I used to be able to hold >7PSI pressure on like only 1 in 10 hard pulls with the OEM valves; the other 9 would flutter and lose the boost. So I think being on the "safer" side for me was still the right choice. It is consistent enough performance gain for me.
Perhaps OP or Jon can add any additional context on how much better the yellow springs hold.