PlanetAtmosphereData.CalculateAtmosphereHeight uses a percentage of the surface air density to define the edge of the atmosphere, but for very large atmospheres with a very large surface air density, this could still be quite a large number causing extreme, jarring drag when first entering the atmosphere. One fix could be to change this to be a exposed via XML as atmosphereEndDensity.
Also, AtmosphereData.SampleAltitude fades in air density and pressure over a hard-coded FadeDistance of 5km. This could be exposed via XML so the player could define how large the fade distance should be.
Finally, our gas giants don't support going below 1 bar. The current code assumes that 1 bar defines the 'surface' of the planet, but we prevent them from going lower. This makes our gas giants feel like they have thin atmospheres, because the user never gets to experience the thicker stuff. Ideally, the quadsphere radius should be much lower than the radius at 1 bar, and altitude should start reporting a negative value below that 1 bar height.