Munster secured a gritty 19-13 victory over the Scarlets at Thomond Park on Sunday to jump five points clear of the Welsh region with two rounds of pool games to come next month.
While McGahan was pleased with the result, which owed much to James Coughlan's second half try and Ronan O'Gara's goal-kicking, he said the two-time champions had been hoping for a 'more complete performance.'
"We have a lot of belief in what we can do and what we can achieve," admitted the Australian after the gritty 19-13 success.
"We want to be growing in this competition and taking the game forward each week, especially the way we fought so hard in the previous three rounds.
"Coming here today, we were expecting a more complete performance. The players are disappointed with the performance.
"We really came here to improve what we have been trying to do. While we got through, we recognise we have a hell of a lot of work to do.
"The big thing for us is to control the scoreboard, but also the clock. We have worked so hard for our points and then let the opposition straight back in with three points, field position, etc.
"If we have any thoughts of going any further in this competition, we need to make sure we recognise that.
"You need to give the Scarlets credit for the way they came and played. There was a real conviction about how they wanted to play."
Meanwhile, Scarlets head of rugby Nigel Davies felt several 50-50 decisions had not gone their way, but he believed the losing bonus point his charges picked up keeps them in the hunt to make the quarter-finals.
Davies, whose side finished strongly on the back of a converted try from replacement Ken Owens, said: "We are very disappointed with the result, but very pleased with our performance. I felt it was a level up from last week.
"It was a game which could have gone either way. There were a couple of key moments, things we didn't get right, a couple of unforced errors which put pressure on ourselves.
"There were a couple of big calls in the game, 50-50 calls which could have gone either way.
"For a player, this place (Thomond Park) is inspiring. It can be quite daunting for the officials and you don't get 50-50 calls.
"I think the losing bonus point we picked up is very important - 19 or 20 points will probably see you qualify for the quarter-finals as one of the best runners-up.
"When you look at the pool at the moment there is still a lot to do, but we are still in with a chance."
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