It depends on what your doing with the car. For just driving around you'll be fine with a different set of tires, as long as their both in the same category (i.e. - high performance tires w/ high performance tires, or all-seasons w/ all-seasons). If your a very spirited driver, or plan on autocrossing I wouldn't mix brands.
There will be a difference but it won't be noticeable unless your doing something that requires the full capability of the tires.
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1999 Ocean Blue Metallic Boxster - blueboxster.com
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