Which ecm are you referring to? The Mefi IV or the 555. Merc. never used the Mefi IV. They used the I, II, and III. Volvo used the IV on a few engines. It is an excellent ecm. They use the 555 on several engines.
As for the heads, there is only a little to gain with some port work. The restriction is the intake. If you changed the intake, then some head work will make quite a bit of improvement. I replace quite a few of the 500 and 525 intakes with a setup from Holley. It is a single plane intake with the injector bosses casted in, fuel rails, injectors, 2000 cfm throttle body, sensors, etc. for about $2500. It is worth every penny. Edelbrock makes a similar setup as well.
With one of the above intakes, a bump in compression, cam change, and a little head work, you can make over 600 hp easily.

Here are a few pics of the Holley intake setup.
Eddie