There was a whole 'umbewafenung' plan to upgun a whole series of existing panzers, most if not all proposed by Krupp, but all of these got rejected on sound reasons. This included the Jagdpanther where effectively a 128mm was proposed. The model Matthew made looks very similar to what is shown in Panzer Tracts (20-1 paper panzers) and in a very old publication I have 'German Tank and Antitank', but it's not quite the same.
Almost all if not all of these upgun ideas were in the end dropped for one and the same reason:
Absolutely not feasible.
The upgunning of the Tiger-B to a 105mm gun was rejected because the 2 part ammo would bring down the rate of fire, the ammount of rounds to be taken on board plus it would require a second loader, where space in the turret would be even more limited than with the 88mm gun.
The same holds for the JagdPanther II, upon which this kit is based. The 128mm gunmount would fill up the whole of the superstructure, leaving almost no room for crew and ammo. Also, the vehicle would be severely overburdening the running gear, even with steel wheels.
This is something that keeps coming back in "What if" models, a complete lack of thinking about what is going on inside. Even Trumpeter didn't when they nicked their E-100B turret. There's no way that would fit in that turret, absolutely no way. You can't minimize a gunmount to fit a turret, the turret is designed around that gunmount.