Not sure you are aware Dave but there have been several reports of the 4 cylinder cars running hotter under certain motorway (I believe) conditions after fitting larger after market ally rads.
Thoughts were that the extra cooling available mucks up the cooling hysteresis with thermostat staying closed longer hence higher temps for a while. Solutions ranged from fitting lower temp stat (probably reason for yours) to masking off part of rad intake, although latter seems counter intuitive to me.
FWIW fitting such rad made no difference on my 3.7 V6 Roadster suggesting better match to increased cooling available. Only difference was that fan rarely comes on now.
