The guys are spot on about system being to small for the load. If temps increase above desired point only at speed the radiator is too small. You must be right on the limit if reducing the heat generated from the engine with lower speeds allow temps to recover or drop. The 200 to 210 would not concern me other than it is above the T-stat set point and it can not hold at the average temp. of your engine. When the weather heats up and is 40+ degrees higher you will wish it was a mere 210.
If temp increases only at low speeds but fall at higher speeds then more air flow is the key. It seems your air flow is good but system is at its limit. Watch your
oil temp as a second clue to that happy place. Ideal is to reach 215 as soon as possible but hold there. The higher
oil temp loads the coolant temp adding to system demands. In Phx. with highs in the 115 range adding an
oil cooler can be just enough to unload the coolant just enough to keep her on the T-stat.
It is about impossible to have too much cooling capacity if T-stats are used for coolant and oil. I also check for and find many pulley ratio problems. With all new components like water pump, fans, rad. and so on cooling problems are almost always a design issue. Track only cars can run under driven W/P ratios and restrictors instead of T-stats but not street cars. Water pump pulleys need to be 10 to 20 percent smaller than crank pulleys with system pressure at or higher than 12PSI. Water Wetter and all the chemicals have been of little value to fixing a problem from my experiences. I like good old distilled water and an anti corrosion with pump lube. Anti freeze in what ever amount needed if freezing is an issue otherwise just watch your PH levels and ground everything to stop electrolysis. A ground strap on the rad. and engine also go a long way.