I don't think Cowher did more with less. He had the top defense in the league, a HOF RB, and a HOF caliber receiver in Ward with solid playmaking TEs (Green and Bruner). This was an era when running and D could win championships. Giants did, Redskins did, Dallas did. All heavy D and excellent running games. QBs were less of a factor back then when Byner and Simms and Aikman were winning SBs. Hell Denver couldn't win until they got a running game, and the Ravens were the last team to do it with D and RBs before the QB era started.
Here's a breakdown of a couple of Cowher's teams vs. the Steelers from this past season (includes players acquired via FA or trade).
1995 Steelers
Starters by Round
1-4
2-6
3-2
4-1
5-2
6-3
8-1
10-1
11-1
UDFA-1
12 players drafted in Rounds 1-3, 10 players after the 4th
2005 Steelers
Starters by Round (23considering Kreider/Tuman had 8 & 9 starts, respectively)
1-8
2-2
3-4
4-4
5-2
6-1
UDFA-2
14 players drafted in Rounds 1-3, 9 players after the 4th
Cowher had some all-time players on this team that he helped draft and develop: Ward (3), Porter (3), Smith (4), Taylor (4). A couple of acquisitions that are some of the team's best ever: Hartings and Farrior. A star UDFA: Parker. Future HOF first-round draft picks: Roethlisberger, Faneca, Polamalu and two additional all-time great Steelers: Miller and Hampton.
2018 Steelers
Offensive Starters by Round
1-8
2-5
3-2
4-1
5-1
6-2
UDFA-3
15 players drafted in Rounds 1-3, 7 players after the 4th
Tomlin doesn't have an extensive list of mid-late round developmental players: Brown (6) is the best in the game and Conner (3) had a great first year as a starter. Haden was a terrific acquisition and definitely one of the best in team history. A trio of UDFA offensive linemen starters: Foster, Villanueva, Feiler. Maybe not HOF (yet), but some excellent first-round talent: Heyward, Pouncey, DeCastro, Watt and a developing star second-rounder in JuJu.
Did Cowher do more with less? As a team, probably not, but he helped to develop a lot of mid-round players into stars and brought in the best talent this team has had since the 70s.
Tomlin has brought in some very good players as well. Maybe not as many all-time team greats, but some very good talent. However, only one AFC Championship game appearance when he had arguably the best QB-RB-WR trio in the game along with a excellent offensive line and a missed playoff appearance this year, I can say Tomlin has done less with more (when compared with other teams in the league).