The France international initially struggled to make an impact after joining the club from Montpellier in June 2012, but Wenger kept faith with the 28-year-old.
Giroud suffered a setback when he broke his left tibia against Everton in August, but he has hit the goal trail since returning and took his tally for the season to 10 by scoring twice in as many minutes in a 2-0 fifth-round win over Boro.
The frontman rounded off a lovely move to open the scoring and some quick thinking then allowed him to get ahead of his marker and volley home Alexis Sanchez's corner as the holders eased into the quarter-finals.
Afterwards, Wenger said of Giroud: "He is a different player to the one who arrived here. He has improved his mobility and technical quality.
"The second goal was a consequence of two players responding to a situation quickly and doing it well."
Championship promotion-hopefuls Boro stunned Manchester City by winning 2-0 at the Etihad Stadium in the fourth round, but they never looked like repeating that feat.
Wenger added: "We controlled the game. They had one chance but I'm pleased with the way we attacked and defended with good focus."
The Arsenal boss was also encouraged by Gabriel Paulista's display in his first appearance since his move from Villarreal.
"It was interesting to observe. He has shown concentration, urgency to defend and good pace," said Wenger.
"There's a lot to come from him because he's technically very good at passing as well."
