Interesting.
My 275/60 R 20 are 10.8" wide vs 12" wide for the 305s. , so only .6" on either side.
The sidewall on mine is 6.5" vs 6" on the 305/50. So 1" different in diameter.
Mine only rub the fender liner in front, on the passenger side, just a tiny bit on full lock right turn. For some reason the fender liner isn't perfect there. No rub at all on the drivers side. Not enough rub to bother me but the 305s would be nice.
The Defender tire's section width does not translate all the way to the tread width, and this may account in part for the difference that you mention. While the Defender's section width is 12.4", its tread width is only 10.0". This, combined with the slightly smaller overall diameter of 1.0" versus the 275/60x20 tire size, is most likely the reason why my 305/50x20 tire size does not rub.
Prior to purchasing the Defenders, I ran the 275/60x20 tire size with a set of Goodyear Wrangler SR-A tires, and there was no rubbing. When looking at potential new tires, I did see at least one tire in the 305/50x20 size that had an 11.0" tread width. I did not select that specific tire, even though I preferred the widest tread width possible, as I believed that the increased tread width might result in rubbing.
One final note, prior to installing the Defenders, I purchased some plastic push-pin fasteners, just like to ones that already exist on the bottom of the front fender liners, and I installed them along the bottom of the fender liners, between the existing ones, just to ensure that the fender liners were nicely and evenly and flatly attached to the existing plastic mounting plate. I did not slit, or zip tie, or alter the fender liners in any other way.