After the missed shift, did it idle normally?
or run rough/ stall?
If the latter;
My guess is bent valves, lotsa bent valves.
and the 10 to 15% is the proof.
Normal is closer to 4%.
I've yet to see a reference that says 4% leakdown is normal and 10% being bad. I can see all the valves being bent resulting in fairly even numbers, but most of the reference I'm seeing is like this:
Personally, I'd figure something was up if I was at 20%. When I did the leakdown test, I listened to the intakes (Intake manifold was off) and never heard any air there. All the air I heard was in the crankcase.
I guess I need to go back and do a compression test.