Ironically, Tom Landry, a defensive coach, had three seasons in which his offense was #1 in both points and yards. He never had a defense that was #1 in both categories. Doomsday I didn't do it nor did Doomsday ll.
As dominant a team as the Cowboys were in the early 90's, that defense never ranked #1 in both categories. No Cowboy team thru it's 68 year history ever has. However, there was one team that ranked #1 in one category and #2 in the other. This is one of the greatest ironies of all.
That defense was on the 2003 team, Bill Parcell's first year as the Cowboys Head Coach. The irony? Parcell's favored the 3-4 defense and his plan called for that change the following season. The 2003 team was a "lameduck" 4-3 featuring players that were not compatible with Parcell's 3-4 like smallish linebackers Dat Nguyen and Dexter Coakley, Greg Ellis, who disliked his future roll as an outside LBer, and Ebenrzer Ekuban who left after the season..
That defense is statistically the best that the Cowboys ever had compared to the rest of the league. They held 6 opponents to 7 points or less. Every starter but one recorded at least one sack. They held opponents to an average 353 yards per game.
With that said, I believe you would have to look at the 90's team as the last time the Cowboys had that much potential to be elite. The current D-line rivals the depth of that great Super Bowl team which was built before salary caps and free agency.