will this help?
 

I have a S5 TII and was thinking of upgrading the radiator cap with a higer psi rating, since the one I have now does not seem to do the job.

Problem: On ocassion, especially on hot days after driving the car for a long period radiator fluid spills out from the radiator where the cap connects to the radiator and the coollant in the resevoir tank boils.

Would this help to prevent the coolant from spilling out and the boil?

On another note, currently I have a koyo radiator and am using an electric fan that stays on when the car is running.

a upgraded radiotior cap should keep it from spilling because it is designed to hold more preasure but i would make sure that your radiotor isnt clogged or anything and that the cooling system(radiotor, fans, and thermaste) are all working just to make sure nothing is bad. its better to be safe then sorry

What is the cfm rating of the fan? It may not be enough.

Thanks for the quick reply, what does CFM rating stand for and where could I locate that information? When I purchased the car, it already came with the electric fan.

Cubic feet per minute. You need a fan rated for at least 2000 cfm with your turbo setup.

just checked on the fan and it doesn't mention anything about cfm

It won't. You need to find the brand and model number of the fan, then do an internet search for the specs.