I just did a bit of very down and dirty research on the scores that were made with feathery balls. It seems that, up to 1849 (ie in years when every golfer was certain to be using the feathery) that the lowest score in the R&A's King William IV medal was 101.
Now obviously this proves nothing. But if the best golfers of the leading club in golf could not break 100 with the feathery ball, I suspect that today's pros would not be that much lower.
I've said this before, but the invention of the gutty was possibly the most important development in the history of golf. Before the gutty, golf was a curiosity, a game played in a small number of towns in the east of Scotland. The gutty made golf cheaper and way, way more attractive, and prompted the first golf boom. Without it we would not have the global game we see today.