I prepared mine according to a couple of others at the local autocrosses. What cracks the header is improper preparation to them. I know several guys who wrapped their header with no problems yet.
First thing to do of course is to take them off the car and remove the O2 sensor. Clean it off really good with some soap and water and make sure to rinse it thoroughly. Next cover up the hole for the O2 sensor. I just stuffed it with a paper towel/rag.
You have to spray it down with some high temperature paint. I used VHT or something that was about 1000 degrees or so. I did 3 light coats with about 30-45 minutes drying tie in between. Then you wrap each pipe by following the directions. Making sure it's smooth and stretched tight. When you get to a joint try to make sure you do the joint as even as the pipes.
Then you want to dry it for a day or two - this is the most pain part. I put mine in the garage on a hot summer day. Oh yes make sure there's no rain in the forecast or high humidity.
When it's completely dry you want to spray the wrap with the same paint as before making sure you cover all knooks and crannies. Anyplace that moisture can get into is bad! And will cause the wrap to deteriorate quicker.
Once the paint is dry, add 1-2 more coats. Doing the same as above. 30-45 minutes drying time between each coat. Let that dry overnight.
Then put on the O2 sensor and install on car. Turn the car on and get ready for some nasty smells
I let my car run for about 30 minutes just idling in the driveway.
As far as underhood temps went I definitely noticed a difference on my B16A. I know parts of the engine I would never be able to put my hands on after an autocross run. But now it feels about 30-40 degrees cooler. Exhaust tone did not change at all and neither did the feel of power. But now that the heat is contained in the wrap the radiator and fan and AC are protected from excess heat./