Since the ring has no optical element, why do you suspect distortions? Vignetting can occur, best is to try the ring in the shop, together with whatever filter you want to put in front. Sounds as if you want to reuse your 77mm CPL.
Will look a bit like a trumpet, though. But who cares if the results are good
Many Thanks for the all your advice. Trying to save costs and also don't want to carry like 5 filters for different lenses, so probably get really good 82 mm filters and then step up everything else unless IQ is affected
I routinely convert my travelling lenses to a standard 77mm for ease of using polarisers / filters during trips, and cap them off with 77mm caps ( of course ). The smallest I tried to step up from was 67mm.
For wider lenses do check if the ring will result in mechanical vignetting. Hoods may not fit but I am willing to live with that.