I can't imagine that the silicone wouldn't handle the temperatures, I have a silicone elbow off the turbo output, that gas is a lot hotter than the oil flowing back to the engine ?
The pipe I have is a consistent diameter from the output of the core, through the spigot, down the pipe and into the block. The pipe is perhaps 1mm larger in diameter, I will measure it tomorrow.
If anything i'd guess the standard outlet pipe is smaller (AN6 v AN8 ?).
Yes, I am have turbo problems, not oil related like yours, mine started with a screaming Garret which I swapped for a high quality, hand built,
Turborebuild.co.uk unit, which turns out to be completely unsuitable for a 300tdi - although not in thier opinion - in mine, my defender owning friends and two proffesional Land Rover garages ...
https://www.landyzone.co.uk/land-rover/300tdi-turbo-pickup.372916/
https://www.landyzone.co.uk/land-rover/300tdi-setup-tuning.373994/