I wouldn't worry about it unless the temp continues to climb with speed.
I bet your radiator is fine. People jump to the faulty radiator conclusion if they haven't had much experience with shrouded electric fans.

Chances are you've got too tight of a fan shroud. It's trapping air and not allowing it to flow through the shroud.
The fans seem to be able to move fast enough to keep air moving through them at speed but not fast enough to evacuate the full volume of the shroud.
A quick fix would be to cut some holes in flat areas of the shroud. Cover them with flaps of rubber or hinged plastic hanging from the back of the shroud above the hole.
At low speed the fans will draw the flaps closed.
At high speed the air trapped behind the shroud will escape through the shroud.

I had the same problem when I was running the Maxima fans on the hot rod. Adding the flaps helped cool it down on the highway and did not change the low speed cooling.