I found a fix quite some time ago to the leaking exhaust issues. Here is a post describing what I did:

http://nsr250.net/forums/viewtopic.php?t=10650&highlight=leaky

Main points:
Don't use the orange RTV sealant - use the high quality grey stuff, it's rated to higher temperatures
Use the KTM vitron o-rings
PTFE tape around where the flanges slide in may also help

Mine haven't leaked since following this method. Hope this helps.
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MC21 Honda NSR250. TYGA 300 conversion.