Paul, I think what you're referring to is the heater plate that was fitted between the intake manifold and cylinder head on later models. It's an electrical heater that is used to warm the fuel on cold starts to help reduce emissions etc.
It's fairly restrictive and by removing it you can gain a few more hp.
As far as I can tell, it was fitted from engine number ending -T007132 to engine number ending -V058528. This spans some 1996 and 1997 cars.
It was only fitted to turbocharged engines. I have heard it was only the B234R (manual Aero) engines, but the parts catalogue does not mention this.
From a rough measurement, I worked out it covered approximately 25-30% of the inlet port area. The boost pressure on cars equipped with the heatplate was increased at high RPM to maintain the same power output as the non-heatplate engines so the effect of removing the plate should be the same as that of fitting a mildly-tuned ECU. I worked out once that it should bring the Aero's 225hp up to around 240hp. I certainly felt the difference in the upper RPM range when I removed it from mine.
It sits between the inlet manifold and cylinder head and looks like this: