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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
$29.99
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.