As stated, an open keelboat is entirely different from a cabin keelboat. Just youtube "sailboat broach" and you will see what an enclosed keelboat, even with the hatchboards in, can typically withstand.
An open keelboat is different, because at a certain point the coaming will go underwater and at that point you will get downflooding. Make sure you have positive floatation in the boat!
Tha said, I think you are a little over concerned. 10-15 knots isn't alot of wind frankly, and getting the rail under is probably not unusual even in an open keel boat (I race against a shields and they often have the rail just above the water).
Install a clinometer. It's like $10 and worth every penny. You will know exactly how much the boat is heeling, and it will take the fear out of it. One idea as well is to fill up a dinghy
with water, and strap some lines
under it. Hook up your main halyard to it, and crank it on a winch
until the boat starts heeling. Using the clinometer, measure the point at which the water gets just up to the coaming. That will give you an idea of how much heel is your absolute max in flat water.