I don't think it's the ecu.

The correct timing is 5*btdc with the brown connector set the timing wire grounded. Both the 12v and 24v 3.0 have the terminal on the big harness on the firewall, 24v on the pside IIRC, 12v on the dside. 15* is way too much advance. This much will kill power down low. I think you have a det sensor on that engine, and that may be retarding spark which will also kill power and economy.

Make sure you are grounding the right terminal and reset the timing to 5* and see what happens. If you are seeing variation in the timing with the connector grounded, either you have a bad dizzy/crank position sensor, or you are grounding the wrong terminal. After correct setting, you should see around 15* and it will jump around, and may vary 7* or so, with the timing connector ungrounded...