You sure you want your "regular garage" to base an estimate on the "shop time" rather than the actual time? Sounds like this is their first Boxster clutch time. Could take more than the "book time".. will they charge for this? How much would a Porsche dealer charge you? (at least they should have the correct tools, parts and knowledge). Not saying it is rocket science but...
Why not just tell him its a "4 hour job" and you won't pay more if it takes longer.
