The EJ head and piston combustion area is far superior to the ef-el and can take a lot more combustion pressure. Even better is you have the head off and radius all edges. The piston rim goes close the head for good squish which ensures the combustion flame front is centralised. This means less timing advance is needed. With the head spacer you will loose all of this. If I had to go lower compression I’d be taking material out of the head to deshroud the valves to the max.
While rewritting the std ecu is beyond anything I’ve looked into, I “think” it has to few variables to be bothered with it at the limit. A old motec, haltech or autronic would be less than the price of having an head spacer made - or perhaps you can make your own spacer?
What you are doing is cool and I look forward to hearing the progress.