Believe it or not (and you probably won't until you do the sums) an oversized ballscrew will be a bigger limitation on performance than, say, the weight of the gantry. Especially a 5mm pitch as it has to spin faster than, say, a 10mm pitch, for a given speed. When the gantry accelerates from rest, you have to spin up the ballscrew and the rotational inertia of the ballscrew will be a major factor slowing it down. Ditto fitting a too-big motor - a combination of inertia and other electrical factors will reduce performance. This is why it's important to take guidance from the community here - use their experience rather than paying for your own!