I have used the M800 on turbo engine's, although that were Porsche turbo's.
I did use a wiring similar to the Cosworth engine and even used the Cosworth amal valve (boost control solenoid), which happens to be a standard Porsche part as well.
I had no problems getting it up to the required boost.
I used a new wiring loom from Motec and made that fit the Porsche engine.
There's no need for the original ecu to be in the car, not with the Porsche and not with a Cosworth engine.
Wiring can be found on the Motec site.
See:
http://www.motec.com/m800/m800downloads/