Many MGC engines have a casting flaw in the passage from the water pump into the block. This is a view of the front of the block with the water pump removed.
This casting seam material reduces the water flow into the engine.
If you assume this passage is about 7/8″ in diameter, then removing the casting seam material would increase the diameter to about 1 1/8″ diameter. This is a 65% increase in area or flow volume.
Worth doing, especially if are already changing the water pump. This was accomplished with a die grinder and an extended shaft carbide bit.