His car isn’t overheating. He probably doesn’t even need new fans. Maybe a more accurate temperature gauge and a better controller. But the car isn’t even really running hot, so recommending new fans isn’t necessary at this point.
Fan diameter is only a small part of fan performance anyway. Cfm rating is what’s important, not diameter. A good shroud will pull air across the entire core regardless of the diameter of the fan. Spal makes 10” fans that pull anywhere from 749 to 1103 cfm. Perma-cool makes a 10” fan that’s rated at 2,750 cfm. If you just look at 10” fans from Summit, they range anywhere from 600 cfm to 2750. Two 600 cfm 10” fans wouldn’t cut it. Two 2750 cfm fans would be more than enough. There’s a lot more to fans than diameter.
Fans, Electric - Single Fan Quantity - 10.000 in. Fan Diameter (in)