Offset has nothing to do with how streched the tire is. Offset will widen the stance of the wheel on the car, but not affect the tire.
I 55 series will be a taller sidewall than a 50 series. If you are looking to get that streched tire look, run a wider wheel, narrower tire, and lower profile tire.
Keep in mind, these extreme tire streches are cool to look at, but make the tire very fragile and it will not operate as intended. A sidewall is part of the casing structure and designed to function at a certain angle. This is why most tire application guides have a "suggested wheel width" section.
