I think I'd be inclined to take it with a pinch of bollocks (ouch!).
I'm not sufficiently au fai (s'cuse my French) with chipped injection mods myself (I'm a bucket for a carb and huge cubes to pour it down man myself) but I'd be very much surprised if Porsche had left some 20 HP carelessly lying around for some Johnny come lately to 'find' with his box of magic beans. I can just about imagine releasing a little extra from a US import (I believe they're slightly detuned to cope with strict US emission laws) but 20 HP ??? from a Brit motor??? He's having a laugh. You'd be lucky to pull that out of the hat with serious valve work, porting, lightened rods, boring out and expensive injection system upgrades, better breathing and tuned exhaust.
If his chip works he'd be marking it up nearer his competition, say, $50 less to undercut them. He's not. Draw your own conclusions.
The anonymous yahoo contact address shrieks dodgy too.