Did you try using a fresh fill of genuine Porsche transmission fluid? I found significantly reduced shift effort when I changed the fluid in my ~120k mile 5-speed Boxster. Most noticeable in lower gears, and when cold. There are different Porsche fluids spec'ed for the 5-speed and the 6-speed transmissions - I'm not sure how big a difference it makes if you use the wrong type, but I used the fluid (the latest/superceded part number for it, anyway) that Porsche specified for the 5-speed. Got a very good result with it. No history of any transmission damage on this car, (at least that I'm aware of), and I also can't say for certain what the old transmission fluid was in there that I drained out but I hope that helps.
Transmission gear oil change made easy.
Cheers!