Being taller than the optimum height is still better than being shorter. Del Potro managed to win a slam and Berdych was close. However, there wasn't anyone at 5'8" was ever a threat.
Ferrer and Davy can't do much with just talent alone. Had they played in the 60s when height wasn't an issue, who knows how much they would have accomplished.
RF 17(4-1-7-5)-6-5-23-81-302, RN 14(1-9-2-2)-0-3-27-64-141, Sampras 14(2-0-7-5)-5-6-11-64-286