First off a 245 will fit on a 7" rim no problem.
Second. The Boxster understeers in slow speed stuff like on an autocross course, so you need to change the balance between front and rear grip with proportionally more grip on the front. The best way to do this is by increasing the front tire width relative to the rear.
If you were only going to autocross on them then I would go with 255 all around. But that combo will be a little tail happy at high speeds. For a tire that you are going to street drive as well then I would recommend either
235 f / 255r
or
245 f /255r
Looks like Falken makes a 235/17, 245/17 and a 255/17 in the ST-615. By the way, the ST-615 has very little hydroplaning protection; be careful in the rain!
Regards,
Alan
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