High velocity in that 40mm is compared to other grenade launchers, not cannon sized calibers. As UkroTurk mentioned, 40mm MGL is pretty slow.Would smaller diameter mean any of that? We have high velocity 40 mm grenades and 30/35 mm bullets, I don't see what difference 30 mm would make.
35mm gun, in this case the GDF or its local clone is a very long and bulky gun and the feeding system is also pretty big, so the lightest 35mm turret you can have from it is Korhan or Göker sized.
Venom LR or a substitute of it is best of both worlds and can be fitted on a STOP sized turret on a 4x4 vehicle, hence its main advantage, if it works as advertised of course.
40mm MGL systems like Aselsan’s Şahin still has a place on the battlefield, mainly on base duties, but a 30mm gun like this is much more mobile.