I don't know much about 911's...but on 914's the area between the front strut towers is STRONG! I've seen pictures of the cutaway and it is multiple boxed layers of thick metal (I can dig them up and send them to you if you want) The only reason to put any kind of connection between the struts would be to make camber adjustment easier. As far as the rear, the braces that bolt in the trunk are a waste of time, there isn't much lateral force on the shock towers and the rear firewall is also multiple layers of thick metal. Porsche got those areas right.
If you want to stiffen the car up a good weld in cage or well designed rollbar would do the most. The weak areas on the car are by the front and rear of the doors.
Let me know if you have any more questions...I've spent a little time with 914's