I have a 3.0 in my car and use the stock gtv6 radiator. I have not had a radiator-related overheating problem and am not aware of a general issue with the gtv6 having a radiator capacity issue. Coolant tank cracking seems to be the biggest issue.
I live in NJ and it is reasonably warm here in the summer. @Gepetto
's car is in NY. I do see my temps on a wonderfully inaccurate gauge system going up and down in a narrow range but nothing concerning. I have to give the dash a little tap to get the temp gauge off zero when it is cold, oil pressure always reads high, fuel gauge response not linear, tach and speedo needles hard to tell since the tips were clipped after twisting in hot weather. LOL
Milano 2.5 stock radiator does seem insufficient for hot weather and a common replacement is to use the Milano 3.0 radiator with higher capacity. I am pretty sure Milano radiator is not a direct fit for the gtv6.
I was not thinking that the radiator here is an issue although a "new" radiator might have a blockage in it or not flow as well as the original one, resulting in the system not cooling as well with fans off, and faster cycling with them coming on and off. Perhaps the t-stat is not opening fully now. I even thought that perhaps the fully shrouded radiator does not cool as well with the fans off since the airflow is now pushed through a smaller area and between the fan blades, versus the stock configuration without a full shroud. But the description of the gauge behavior seemed pretty normal to me for a warm location and normal driving.
, would be interested to know why you replaced the radiator and chose a new radiator instead of having the original one repaired (if it was leaking or choked up)? I presume you had an original fan die and did the SPAL replacement?