I wouldn't worry too much about what looks like losses below 3000 rpm, dynos are notoriously inaccurate below 3000 rpm and much of it depends on how the operator puts their foot into it or not. Is this the same
plenum and throttle body for the Cayman S which comes from the GT3? If so, then I'm not surprised that on a 2.7L Cayman that it simply is capable of flowing more air than the engine can handle. I also concur that once you free up the exhaust you should see some improvements across the board. Interesting that one dyno shop shows no gain and another one shows gains, just goes to show you how you can't compare dynos between shops (which people always try to do here) or between cars, etc. Good to have a record for yourself to compare future mods against, and that's really what you should use it for.