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.
Federer > Davydenko > Nadal > Federer
Nadalgaenger: "Djokovic is clearly better than Pete"