Before you assume that it is a bad engine or transmission mount, check your suspension. The rear trailing arms make a loud banging sound when going over sharp bumps (very common), but it could also be sway bar end links, front trailing arms, or the control arms themselves. The rear trailing arms are pretty easy to replace.
How many miles on the car? 60K - 80K is typically when the suspension starts getting noisy.
Is your mechanic friend really familiar with Boxsters? No offense to him, but if he doesn't work on these cars often, it may worth taking the car to an independent shop that specializes in Porsches.
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