He's asking how bad will it be if he buys a ski that is 3 cm above his head instead of one that is 4 cm below his head as recommended by manufacturer.
He says he knows it would not be troubling him for off-piste, but he is going to be skiing on piste.
My answer is it depends on how you ski.
If your main means of turning your skis is to tip them up onto their edges and let them turn, then the longer skis won't be that much trouble. The tips being further away from you will exert more torque and be less forgiving, less easily overpowered by brute force if you make a mistake and will force you to learn to be a better skier. They will be slightly less nimble and maneuverable, but will reward you with a little added stability in the fore-aft plane when you go over bumps or crud piles. Also because your weight will be spread out over a longer length, they may have less bite on hard snow due to less pressure forcing them down into the snow.
If your main means of turning is to pivot your skis to a new direction, then it would be a mistake to get the longer skis. They will be much harder to manage, refuse to turn for you and wear you out.
I'm my opinion you should just get the longer skis and learn how to ski, but that's the hard road.