The character strikes as an invisible creature (+2 attack), denying the character's target any Dexterity bonus to AC.
Sneak Attack: Any time the rogue's target would be denied a Dexterity bonus to AC (whether the target actually has a Dexterity bonus or not), or when the rogue flanks the target, the rogue's attack deals extra damage.
Even improved invisibility you'd still think people would get some advantage after the first attack
Logicaly I still think it should be harder to sneak someone who knows your there. but yea it would be hard to represent.