Sandbagging is a joke but theres not much you can do about it. Just go out and play the best golf you can play and be happy with whatever results come from it. Playing in invitationals or any tournament for that matter is so much better for your game than just playing 18 with your friends. It will also expose you and put you to the test real quick. Be careful to criticize any tournament scores if you're not willing to play yourself. I would say the average amateur would see a 5-6 stroke increase under tournament conditions.