They are suppose the be restricted. You just want to have a bit of flow to prevent steam/air pockets. The coolant flow goes from front of block to back of block, up through back of heads to front of heads to thermostat.
Incorrectly enlarging the holes can show a lower thermostat temperature, but the back of the block will be running hot because you are bypassing the flow to the back of the block.

The exception to this might be increasing the hole to the next to last cylinder to help cool the center exhausts area of the head.