Acceleration will increase significantly and the last gear will be normally usable. We guarantee you a real increase of your speed cruise.
A 150 or 180 cylinder will not increase more the maximum power because you cylinder head is made for a 125
This is why we don't increase so many the volume
A big cylinder give more torque but after 3/4000rp/m the power is no more than the 125 because the cylinder head become too much small for a so big cubic
At 8000rp/m it become catastrophic and can give les maximum speed than a 125