That's a good point and exactly why timing is based on individual engines and how it was built. It sounds like in this case you have a high duration cam with reduced manifold vacuum at idle. Vacuum is also increased by raising the idle other than adding more timing. The double edge sword is with engines with radical cams when you raise the idle to far you expose the idle transfer slots in the throttle bores and you loose adjustability in the air fuel circuit. This whole issue is why some after maket ignition systems have a built in retard on start. It makes it easier to start and have adjustability.
I think I would still go with the 10L for 20 deg. of mechanical. With it not working before, this engine is going to be a lot different and I think you will see the power and mileage improve.
Wayne