Your question of concern is common.
Head gaskets are designed to control the flow pattern and volume of coolant between the block and the head. Yes, some casting holes are blocked and/or restricted... again, to control the flow pattern and volume of coolant.
DO NOT EVER modify the head gasket's holes or you will encounter problems with engine temp at areas of the engine that you can't see. These patterns are designed by the OE developer.
I'm a former long-term applications engineer for a major gasket company.
The slots prevent air pockets and allow a limited amount of coolant flow to help equalize the cylinder head temperatures. The majority of coolant is forced to flow from the front of the block to the back of the block, then up through the back of the cylinder head to the front of the cylinder head.
Enlarging the gasket holes will reduce the amount of coolant going to the rear cylinders and cylinder head.
There is a misconception that making the holes larger makes the engine run cooler because the front of the engine where the temp sender is will be running cooler than the rear part of the engine.
