I personally would not drill a hole larger than 2-3mm if at all. My advise would be to go to Burson's get the correct thermostat that is listed for your car, While you are there get a product by Redline oils called "Waterwetter" This stuff is really good at keeping your engine at the correct temp as per your thermostat. Have used it in many cars from my zook when it had the turbo motor up to 11 second 1/4 street cars that would overheat in town. Bloke whom I sold that one to was over the moon that he could drive in town again.
any way it is good stuff. check the link Waterwetter
After using in my turbo motor I use it in all my cars.
Waterwetter is a corrosion inhibitor but has no antifreeze capability. you will only need about 1.5 liters of antifreeze concentrate.
