ok. not thinking coolant sensor as it should not be warm enough to be in closed loop.
if it is running poorer with higher fuel pressure, then i would suspect that either you are geting waaay too much fuel in the injector pulse, or the atomization is not sufficient, or the dme is holding the injectors open too long (either by mistake or by faulty input)
i would verify there is no other way that fuel is getting to the combustion chamber (charcoal canister), and see if you can get the injector pulse measured. a power graphing meter would be needed, as a regular dvom will not be able to display this info.
not sure what the spec is, but a few milliseconds is pretty much all that is needed.
if it were a coolant sensor, i would expect a difference in running with the circuit jumped out.