The swirl dam is there to promote mixing of the air and fuel in the cylinder. Some head porters do remove them because it allows them to get bigger flow numbers. It will also incread the volume of the combustion chamber resulting in a lower compression ratio. On an NA engine its a bad thing with with a blower or turbo it can work. Personally I would either leave it alone or modify it slightly (rounded corners etc.) but I would not remove it completely.