myring

Forget tail cone design and size (or twin or single) for the moment, you are even able to run it without a tailcone at all. Yes tailcone
will effect temps but they are much more related to getting more thrust from the turbine exhaust gases.

Blue as found is a good indication of very hot running. The likely fault is an uneven burn from an uneven or blocked manifold. If the manifold when tested on gas set up (see pic http://www.rcuniverse.com/forum/m_29..._3/key_/tm.htm )
Note the needles are clear but the flames are not even height and need to be corrected.
The fuel needs to be directed towards the hot face of the sticks to vapourize quickly and evenly.

As Shadow9926 stated
" Idle is often when the EGT is highest, but your temps are too high. If you can notice any hot spots on the tail cone at idle , then I would suggest that you may have an uneven burn problem, either to do with the combustion chamber or a blocked fule manifold (more likely).

I have seen blocked fuel manifolds cause engines push the flame out of the exhaust and the needles on the JJ-1200 are only 24g so any small amount of dirt or flux from the soldering process could be blocking one or more needles"

THere are other reasons for the tailcone to glow bright red but not on the latest JJs.

How old is yours??

Then if all is fixed and corrected you can use my earlier post on the FADEC set up and training which directly helps temps to achieve cooler running (only with a very good turbine pump, buy the best and filter fuel regularly.)

Paul