Panzerschiffe armed with 12-inch guns

Would the Panzerschiffe be better off with 305 mm guns instead of 283 mm guns?

No; the change offers little practical gain and may only encourage Britain to build Vanguard-class battleships sooner.

Moving from 283 mm to 305 mm guns does not change the class's small battery or lack of speed. The ships would still be very heavy cruisers without enough guns or performance to punch above that role. The most important consequence may be political perception: 12-inch weapons make them look dangerous enough to accelerate British construction. An earlier Vanguard program would leave Germany facing a much stronger opponent than any marginal gain in shell weight could offset.