I'm not an expert but I had a failed water pump once on my 320. It caused the coolant to bubble through the rad cap due to the excessive temp and pressure. Although I have a european car I'm sure yours is similar, and would say it does act as an overflow.
The line on the rad states cold level, after you've blasted down the highway the water warms up but it shouldn't foam through the rad cap since the cap will hold in any reasonable pressure.
If I were you I would bleed the coolant system a little (5 min job) just to see if there are any serious airlocks. There is a bleed on the top of the rad (black plastic crosshead) and also a 17mm or similar on the exhaust side of the engine block, easily accesed from beneath the car. easwe of the bleed bolt and let some coolant flow out (only a little is required). Hopefully you will here a hissing noise as air is escaping. These motors are bar stewards for airlocks if not filled properly.
Always fill your rad very slowly too, will prevent air locks.
I think that no matter how hard you drive it shouldn't overflow (but i might be wrong).
Also make sure when your filling from empty the fans have been left in the hot and on position in order to open the solenoid valve for the heater matrix.
An airlock may warp your head if its stopping flow to the head but I doubt this is the prob, more likely something smaller. run the car till hot and Sqeeze all the hoses you can see. If they are solid (feel like a over inflated tyre) youve found your lock. A normal coolant hose will still be squidgy upon sqeezing but a air locked hose will be closer to rock solid after a good run, but be careful.
Pete (the ews ratsnest)