For example, revised cylinder heads. Additionally, revise camshafts and tuning to go with them. Finding an aftermarket source for these parts with comparable tuning to make them work is not easy. They certainly will not be tested, engineered, or developed to the level Porsche did it themselves. While $17,800 is a lot of money you get a package that is tried and tested with a warranty so it has its appeal. Many will argue one should take that money and just get a GT3 but the Power Kit option has existed since the 996 days and some people prefer the S or 4S models rather than the sharper GT3.
The article below explains the Power Kit better than anyone, well worth the read.