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I feel your pain as I went through the same thing. I think that due to the directness of the suspension and performance orientation of the car there is always going to be a little bit of vibration. I've owned my car since new in 2003 and there's always been a little steering wheel vibration. I find that the car is sensitive to the road surface, amount of gas in the tank and tire pressure. Do you have Porsche Boxster OEM wheels in a 16, 17 or 18 inch diameter with the proper offsets? If you happen to have 19 or 20 inch wheels with even lower profile sidewalls that could be part of the problem.
Although you have new tires, I think you need to see if the issue can be isolated to the wheels and tires. Do you know anyone with a similar car or a shop that can help you out? If there is someone who has a car without this issue, see if you can swap wheels and tires and drive both cars with the wheels swapped. If your vibration goes away and it appears in the other car, the vibration's cause is in your wheels and tires. If the vibration is still the same with the other wheels and tires on your car you've isolated the issue to something in the car. If this vibration went away with the other set of 4 wheels and tires, while this is risky and it would be up to a shop to decide if they would do it, the wheels and tires could be swapped back one by one with test drives in between each swap to isolate it to a specific tire. It may well be that no one will be willing to do this because of potential handling issues with mismatched tires which is understandable. If you have a friend with a Boxster but not the ability to swap wheels, maybe you both could take alternating test drives in both cars. You may find the steering wheel vibration in your friend's car feels exactly the same, better or worse. If it feels better, than that could be a reason to keep trying to track down the source of your car's vibration. If it feels the same or worse, maybe you've reached the point where it's not going to get any better.
Another suggestion is to take your car to a shop that does high end alloy wheel repair and refinishing. They can unmount your tires and put the wheels on a machine or jig with dial indicators and precisely check all of your wheels for any out of roundness, and any radial or lateral runout (up and down and side to side wobble) on both the inside and outside rim bead. And if they find any, they can true the wheel(s). You said the rims have been checked for bends, but not with the tires off. It may be that these people eyeballed your rims as the wheels spun on the balancer and called it good, or maybe even held a pencil next to the spinning wheel to look for issues. For a lot of cars that is good enough but I know that it's not enough on mine with 18 inch Lt. Carrera wheels which bend fairly easily on what passes for roads around where I live.
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