I can't imagine why a car Porsche Piwis Tester II without a sunroof would be more collectible than one without, especially a Porsche. With 356's and 911's (real porsches), sunroofs can sometimes double the value (Case in point, I know of two '63 356's. Both are smyrna green, with the same option packages. condition and general mileage. Both run and drive, and have decent paint. The slickback went for $35k. The sunroof car went for $70). That said, I never Porsche 2 really considered the 928 to be all that collectible, as the engine is water cooled, flawed in it's design, and has too many cylinders. Don't believe me? Compare the value of a 911 and a 928 of the same year in roughly the same condition. That isn't to say that they are necessarily bad cars, they're simply bad Porsches.