Kuhmo 712s are great tires. Great grip, good wet weather traction also. A lot of people use them for autocrossing. They have really good grip for the STS classes and a lot of stock classes use them also. They wear pretty well also. Only downside I've heard of is a little more noise, but at our tire size they're all loud.
It's a pretty good deal at $79 a tire. If it's mounted and balanced also, Go For It! Are you replacing your old dunlops? How much tread depth is left on them? I might be interested in using them as race tires if I can get some new rims cheap.
The 40's would be good for autocrossing to give you a little better acceleration, but for normal driving, you would need to be careful, and your speedo will be off a bit.