I've been tracking down supergun discussion in Chinese social media which resulting in me to finding this article:
This is an aerodynamic analysis for extremely long range guided artillery rounds, the proposed round, intended for a 300mm gun with the goal of maximising range looked like this:
Muzzle velocity out of the gun is 700m/s. Projectile weight 530kg with 230kg of filling (surely this is either just the solid rocket fuel, or rocket fuel and explosive filling?). Launch angle is 55 degrees. The rocket assist kicks in 10 seconds after firing and provides a further 1000m/s or so during flight over a 40 second burn time. Maximum altitude reached is 55,620m. Before reaching maximum altitude the shells flies in an unguided mode with the four small canards undeployed.
The paper at this point simulates two scenarios:
1. Unguided flight - the canards remain undeployed and the round flies under pure inertia
Velocity over time:

Ballistic trajectory:

The round ends up travelling 280km over 241s of flight. Impact velocity in the area of 1200m/s
2. Guided flight - the canards deploy and maximises round glide ratio with an AoA between 5 to 8 degrees

In this scenario the round flies 420km and impacts at about 297.8m/s, thus trading impact velocity for a 33% range improvement.
Scaling this up to a 400mm gun, adding some other innovations (ramjet, better glide vehicle, higher muzzle velocity etc) and maybe 1200km could be reachable.
300mm gun with 420km range already has some interesting applications.