"In my opinion there are only a couple of teams in Europe that you could class as a complete side - and Leinster are one of those teams," said the Springbok powerhouse.
"They have a good pack that can dominate, talent out wide, and the experience and the know-how to win games.
"They are a big European side who have represented themselves well this season, as they have done over the last couple of years.
"We all saw their quarter-final win against Leicester Tigers, it is on all of the computers down at the training ground, and now we will sit down and fully focus on the match and on Leinster.
"I thought they played really well and deserved the win - it really was a big statement beating Leicester.
"However, Leicester were unlucky in the game because they did have chances and opportunities to take the game but didn't, so they will be disappointed."
Toulouse and Leinster know each other well, the French giants 5-3 ahead from their eight meetings, including a 26-16 semi-final win at Le Stadium a year ago, and Sowerby is wary of home advantage tipping the balance again.
"We are extremely familiar with them, we have met them a lot over the last few seasons so we know what they are all about," said Sowerby. "They are technically spot on, they play with pace and intensity and we have to be ready for that.
"Home advantage is huge at this stage and of course the fans will all be right behind Leinster but as the away side we will have to thrive off the atmosphere and make the best out of it.
"I have never been to the Aviva but from what I have seen I'm expecting a big stadium with a great atmosphere.
"The matches that have been played there look great so I'm sure this match will be another great occasion.
"You look forward to playing in a stadium when there are 50,000 plus supporters screaming, whoever they are supporting. It just adds to the event and the occasion and lifts everybody.
"Obviously we beat them at the same stage last year but that was on home soil and home advantage in this competition is huge and we are really up against it in Dublin, it's a huge game and a huge test for us.
"No-one is looking any further than the semi-final stage, we have got a massive obstacle to overcome in Leinster and that is our focus in the tournament. If we manage to get the victory then we will turn our attention to defending the title.
"Biarritz are a top team, they do not always play the prettiest of rugby but they certainly know how to win games. We played well at the start of the quarter-finals but fell off the pace in the second half, though to come through the game was fantastic."