How effective is a slow carb diet with a cheat day?

It depends. It depends on how much you "cheat" on your cheat day. The important part is to think not like something you will do for a period of time and then you can stop. The changes you made to your diet have to be permanent in order to maintain the weight loss. If you are thinking about the day you are going to stop eating healthy food, you have already lost the battle.