One of the upper plenum drains was discontinued on later cars but as far as I know there has always been two lower plenum drains. The upper ones are just under the bonnet near the back of the front wings the other two are below the scuttle panel one below the pollen filter and the other underneath the servo. the latter is only really accessible from under the car.
The key to the garage statement is in the wording "to lose up to". this statement means nothing as "up to" is zero to 35%. this makes it compatible with all above statements. I would expect minimal loss of up to 10%. (see what I did there
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