I decided to get into it. The significance of this stat is questionable but it does tell you something about the strength and depth of the field and the balance between the dominant players of an era (just look at the last few years
The numbers also include players who only became Slam Winners at a later stage, unless stated otherwise.
Most Grand Slam Winners in the Round of 16
US Open 2001 - 11
(Kuerten, A. Costa, Kafelnikov, Hewitt, Safin, T. Johansson, Sampras, Rafter, Agassi, Federer)
8 of them were already slam winners at the time (all but Costa, Johansson and Fed).
With 7 QF titleholders, this also holds the record in that department, hence my assumption was right
US Open 1978 - 10
(Borg, Ashe, Gerulaitis, Kriek, Teacher, McEnroe, Vilas, Tanner, Connors, Panatta)
Again 8 "current" titleholders (Kriek, Teacher won their slams later).
US Open 1976, 1980, 1992 - 9 each
Also worth mentioning are US Open 1969 (11 Slam Winners but only 6 of them won in the Open Era) and Roland Garros 1969 (10 and 7 respectively).
No Grand Slam other than the US Open ever had more than 8 players in the round of 16 that had won an Open Era GS title at some point of their careers.
Least Grand Slam Winners in the Round of 16
Australian Open 1973 - 1(!)
The nadir of the "down under" GS tourney that was of secondary importance in that period. A few others in the 70s and early 80s had 2 Slam Winners. Note that the round of 16 in this case was in fact the 3rd round. It wasn't until 1982 that they switched to a 7-round format and only in 1988 to a full 128-player field.
Wimbledon 2002 - 2
A crazy tournament that saw only two of the top 16 seeds make it to the 4r (the other one was not Krajicek but local hero Henman with a usual SF loss).
Wimbledon 2006 - 3
(Federer, Nadal, Hewitt)
US Open 2005 - 3
(Federer, Agassi, Hewitt)
Australian Open 2007 - 3
(Federer, Nadal, Roddick)
Not much explanation needed here. Of course these numbers may still grow if somebody manages to sneak by the Fed-Nadal iron gate.
As can be seen, the US Open is clear first in this measurement. The Australian is last because of its "isolated" period, but nowadays everyone plays all of them so the numbers have evened.
In more than half of the tourneys the number of GS Winners in the round of 16 was either 6 or 7.
(and, of course, Sjengster's GS Club
was also of help)