I ran 547r's on my first 997tt and 591r on my 2nd 997tt....On HRE's website it indicates the 900 series was brought about for sports sedans and GT cars and i dont see many ppl running these on sports cars...Is that because they are much heavier or is there another reason they are marketed more towards..Bentley, MB AMG sedans etc....

Lou

 

You are correct. The 900 series was built for heavier vehicles and as such is not well suited for a lightweight sports car.
Take a look at the HRE 790 series. It has similar styling but is built for lighter vehicles.