I don't think they would be "too stiff" like your car is going to fall apart because of the additional stiffness, but I think that 23 mm is plenty for the front of our cars and that a bigger front bar is going to introduce additional understeer and additional inside front wheel lift that you don't want.
If you want a true expert's opinion on this stuff, contact Steve Weiner at Rennsport Systems. He's incredibly knowledgable regarding the suspension setup for 911s and a great guy to boot, but be warned, talking to him may cause you to spend money
Cheers,
Jeff