Zane,
How thick is the ring on the variable system? If it's any more than 1cm, then, you will see the ring of the filter on your wide-angled lenses.
I have a variable system with a built-in warming polariser and it's about 1.5 cm thick. On my 17-40mm lens, I have to zoom to 24mm before the ring of the filter is not visible in my images. I think I paid extra and bought the thin version, too.
Unfortunately, there's no way around it, other than using hand-held filters or a filter holding system with large format neutral density filters.
This is an expensive hobby.
H