Excellent specs Martin.

Its difficult to build an effective flipper 1st time....... you look around and then buy an expensive high pressure high flow solenoid only to find that its opening time and flow is nowhere near good enough for the application of robot throwing .......

An OPEN full port ball valve has the best flow rate possible.... the quicker you open it the sooner you reach maximum flow.

Your set up sounds v.good.......but have you tried WITHOUT a weight to flip?
The biggest problems H.P. flippers have found is stopping the arm without it either bending or the inertia taking the robot right over......Oh! whats the self righting like?

Woody