I would change my vote to slightly narrowing. When I voted, I was thinking long putts where the make percentage is low. But it does raise a question. Certainly at 5ft or maybe 10ft, the gap is narrowed because it could only hurt the better putter. At what distance does it start to not matter? Like at 30ft, chances are already low of making a putt amongst the best. Adding in bumpiness is not going to make much difference between the two skill levels. So at what distance is the breaking point?
Intuitively, I believe that it would hurt both golfers, but that it would hurt a good putter more. Hence, the gap would be narrowed.
From 8 feet, a tour level putter would make about 50%. Let's say a mediocre amateur would make about 33%.
If a bumpy green deflects one third of the 8 footers that should have been made offline enough to turn them into a miss, then suddenly the elite putter is making 34% and the mediocre putter is making 22%. The gap has shrunk.
The second consideration - what happens to the putts that would have missed? - is a bit harder to pin down. When you consider that a mediocre putter has more opportunities for their bad putts to be deflected towards the hole, it is seems likely that the gap might even shrink further. But I'm not sure that this is true.
The mediocre putter who misses 67% of their 8 footers on perfect greens should have more opportunities for fortunate breaks. But with that said, a mediocre putter is going to have misses that are so far off line that no amount of favorable bumps can save them. Let's say 20% of their putts are hit so badly that they have no shot whatsoever. By contrast, an elite putter should always be somewhere around the hole, which means that even though they have fewer misses, almost all of them have a chance to be "saved."
In summation, I think the effect on putts that would have missed on perfect greens is pretty much a wash, while the good putter is badly hurt on putts that would have gone in. Therefore the gap narrows.