The effect on the rad-box on a system depends mostly on the tubing used in the system itself. If you are going with a performance setup with 1/2" inner dimension tubing/fittings throughout the system changing to a 3/8" inner dimension tubing (required for the rad box) will be a significant reduction in total flow. If you have no angles or sharp turns the actual length of additional tubing will not effect the overall system that much- adding an additional 6" to a 6' total tubing length system will not be that big of a drop especially if you have a pump with a decent 'head'. If you are basing your system on 1/2" OD-3/8" ID tubing then the rad-box will not have that much of an effect on your system.
Also keep in mind that most 1/2" ID tubing systems actually use fittings that have a 3/8" inner dimension anyway so many times the fittings are the true restrictions on the system. Some people use a 'softer' tubing and are able (via heating in hot water) to stretch 1/2" inner dimension tubing over 5.8" outer dimesion fittings and by doing so open up the fititng/tubing connections where most systems see their greatest restriction.
If you resort to math (perish the thought
then the inner dimension areas of each tubing cross-section are as follows:
A = (pi)r^2
3/8" ID tubing cross-section area = (pi)(0.1875^2) = 0.110 inches squared
1/2" ID tubing cross-seciton area = (pi)(0.25^2) = 0.196 inches squared
Using a fitting that will restrict the ID to 3/8" is right about a 44% reduction in the cross-section of the flow path. Its definitely significant.
If you are using 1/2" OD-3/8" ID fittings then the rad-box should have very little effect on your system considering that its restriction probably matches the restrictions forced by all other fittings in your system.
***CAUTION***
I am a newb to WCing myself so this is only a guess but I think its pretty accurate.
