I've tinkered with my slip fit connection 3 or 4 times. Last time I used a pipe expander to make the fit as tight as I reasonably could. I sealed the joints with some "exhaust sealer" that I picked up at a Napa store. I then wrapped the connector portion of the header/side pipes with exhaust wrap to hopefully keep the sealer from blowing out. It has held up well for about 500 miles now.
I've tried the high temp silicon before and it burned away in no time. I think your measurement of 685F might be in the ballpark at idle but I suspect the temperatures actually get much higher under heavy load conditions. If I do this again I might try one of the high temp sealers I found on this site:
http://www.cotronics.com/vo/cotr/index.htm
Chris