Flaw was the middle piston thing was too large and the boost pressure was forcing it open. Fixed by making it smaller and making the spring stiffer.
Flat is too strong a word, as on boost performance is unchanged. Lethargic when applying the throttle is a better description.
As for the earlier question about flutter, i don't get any with mine. Took this snippet from an early morning blast yesterday over the Black Mountain Pass. Cam was mounted in a waterproof case on the bonnet: