Melvyn:
I think you make (if it is indeed the argument you are making) a good case for Old Tom's win in the 1862 Open Championship being the equal, or perhaps better, than Tiger's 15-stroke in at the US Open at Pebble. Old Tom, as I'm sure you know, shot 163 for three rounds at the Prestwick links to better Willie Park Jr. by 13 strokes (176). With a per-round average of 4.3 strokes better than Park, one could rationally make the case that Old Tom would've won by somewhere around 17 strokes, bettering Tiger's record.
Of course, the obvious caveat is that Tiger faced a much deeper field than Old Tom faced in 1862, when the Open Championship had only seven other competitors, and only Park was arguably a golfer of the caliber of Old Tom (Park was the only other competitor in the field to have won the Open Championship). Tiger beat 62 other players (just counting those who made the cut), and those included the likes of multiple (or eventual multiple) major winners Els, Harrington, Faldo, Singh, Goosen, Mickelson, Olazabal, Watson, Price, Irwin, Cabrera, Janzen, and O'Meara (along with Furyk, Lehman, Mize, Kite, Sutton, Toms, Weir, Couples, Campbell, Azinger, and Duval -- all of whom also made the cut and were/would be single major winners).
One might argue Young Tom's 12-stroke win in 1870 at Prestwick over three rounds (149-161 for Bob Kirk) was better than Tiger as well -- on average, it would've amounted to a 16-stroke win over the field in a four-round tourney. And Young Tom, albiet facing a field of only 17 competitors, went up against a deeper field than in 1862, including his father, Park, and three-time Open champion Jamie Anderson.
Completely different eras, of course, but a fun argument nonetheless.