What is the fine-grained topic of the following text: While a 2.4-litre Porsche 911S, a 993 Turbo and a first generation 997 GT3 RS may not look like they have much in common (in 911 circles at least), after intensive evaluation in previous issues of Total 911, we graded each just half a star away from perfection. Only return the topic.
Evaluation of different Porsche 911 models