Ok, so it makes more sense now that I've done some measuring. The 77mm XS-Pro MRC UV filter on my EF-24-105mm f/4L IS actually measures 80 mm in diameter (the glass is 77 mm, the mount adds to the diameter). The EW-83H hood for the lens has an actual minimum diameter of 81 mm, so clearly they don't count the protrusion of the bayonet mount into the hole when they name it an 83. That leaves only 1 mm difference (which matches the actual measurement of the gap). So, the 'spec' difference of 6 mm (83-77) is really only 1 mm. In your case, the 'spec' difference is 5 mm, which if the ratio is the same, means no difference, plus or minus, and in your case, it's apparently minus.


So, given that, I'd say use the hood, unless you need a filter for effect (CPL, ND, etc.). You can always mount the filter after the hood (at least, it's usually possible with the shallower wide angle hoods).