Remember, this powerhead came from an outboard and there's a lot of support for those still. Mercury has been very good about providing support for sportjet through the years (I've been surprised, but I seem to live in my own world outside this particular forum), not sure the current status but...
Something doesn't make sense, after shutting off while cruising the boat, once restarted, would only idle back to the launch?
How about checking fuel rail pressure?
One of the big tuner outfits can get you an ECM of whatever flavor you want, I'm sure.
This might be the outfit I was referring...
My experience with rings is there's not a lot to gain but in a pro race it might be the edge you need for that 1/10 sec. The ring installed on my Berkeley ruined reverse completely so I took it off and behold the only difference was a vastly better reverse.
So this Yamahaha, 300+hp?