I have to say I don't think it is the rad.
If it works at idle, then higher speeds don't generally reduce cooling.

Higher speed warm up is going to be caused by something else.
blocking air flow would be the first thing I would look at.

for instance a mechanical fan clutched, might be having issues with the clutch. I had one going bad, that allowed the car to warm up quite a bit on the highway.

Or an electric fan, aftermarket, that doesn't allow enough airflow at speed.

Or not enough water flow.
For instance that sucking shut the lower rad hose.
Or a pump going bad, not allowing enough flow.

I would ask if it cools off just fine once you come off the highway.
If so then I would say you don't actually have a cooling issue with your rad.