Frank, I went with th 21/27 setup with Bilstein Sports on my 79SC. It's a perfect setup for mainly street driving with some track time. I would not just put the largest torsion bars on. Remember, that the stiffness is related to the diameter of the bars to the fourth power, so a small increase in diameter will cause a significant increase in stiffness. Also, you can adjust the oversteer/understeer of the car by changing the ratio of front to rear from the stock setup. That is, by allowing more weight to shift to the front wheels, (e.g. select a torsion bar pair that gives more stiffness percent wise to the rear rather than the front over stock values), you could induce more oversteer if this is what you want; or vice versa.
The attached Pelican link explains the relationship between stiffness and the diameter of the torsion bars. What you will notice is that the 21/27 setup gives approximately the same increase in stiffness front and rear, 56% and 57%, thus maintaining the stock ratios. I think this is one reason many go with this setup; stiffer suspension with similar oversteer/understeer characteristics to the stock setup.
http://www.pelicanparts.com/techarticles/911_torsion_bars/911_torsion_bars.htm