What about having a weight that the solenoid holds up and allows to drop, kicking the ball, not sure how heavy it'd have to be or anything, just throwing possible ideas out there (and you may have requirements that don't allow that)
What about using mechanical advantage to increase the speed of the blow delivered to the ball. If the solenoid were attached to the shorter side of a pivoting arm (a lever), then the longer free end of that arm were used to impact the ball, you might get better kicking distance because of the increased speed.Good ideas both, but unfortunately i can't really use any of them as i have a weight and a size limit, i could only maybe put some-sort of metal part on the end of the solenoid to increase the contact with the ball , but even then i wouldn't get a much better shot.
I bought all the required items to test the circuit i posted in my previous comment.
Any other advise and suggestion on how to get a better step-up circuit is welcome!