Okay so I'm currently building g a 20v and I do not want to deal with overheating issues so I'm happy to pay good money for a nice and big koyo alloy rad but I've been told that running one would be so much overkill that the motor will run cold and I definitely don't want that. The car will be driven on the street and thrashed at the track. Opinions on whether or not a koyo rad would be a good option for my setup or if not then any other smaller alloy rad that are also good quality? thanks.

My build specs.
4A-GE High Comp 20v Blacktop 
CP 81.50mm bore high compression forged pistions C/R 12:5:1
Brian Crower H beam forged conrods
ARP head studs
Fidanza Adjustable Cam Gears 
HKS timing belt 
Kelford 194-C racing cams
Intake: 284' Duration 9.30mm Lift.
Exhaust: 278' Duration. 8.50mm Lift
Kelford Cams racing valve springs
Fully Reconditioned Block and Head
Port and Polish
New Water Pump
New Oil Pump. Ported and High Pressure Kit.
TRD 0.8mm multi layer metal head gasket
1zze Coilpacks

Stock radiator works great, even at 112 degrees at Thunderhill. I haven't seen any reason to go away from the stock radiator.

There shouldn't be any need for an aftermarket rad however there should also be no concern of the motor not getting to operating temp. That has nothing to do with the radiator and is completely dependent on the thermostat.
Now yes if you live in Alaska and regularly see -40 temps you may need to cover part of the rad just because the cold temps are sucking so much heat but in normal operation your thermostat is what regulates the temp.

It's not overkill but also not really necessary. Assuming you use the stock t stat the water temps would only be a hair lower than normal. The only time I could see it potentially being an issue is is in a true winter climate while daily driving, but that's just speculation.

The good thing about the stock setup is that it's light due the lower capacity. The bad thing is the radiator has plastic end tanks that won't hold up like an all metal radiator.