Condensation efficiency refers to how well it removes moisture from the clothes , not how much it leaks .I don't know yet as I haven't bought one yet. But considering the outbuilding suffered condensation to rust my tools in the past, would be crazy not to install whatever measures possible to reduce the risk given they'll be hundreds of £ of equipment in there. Of course tumble dryers don't chuck out moisture, but the heat they generate is a cause for condensation in a relatively cold outbuilding, and depending on the quality of the seals there will always be a degree of moisture escaping, hence the ratings of condensation efficiency they come with.
They cause no condensation .