To test I took a 6mm stainless 'bolt'. It's actually another leftover element of the salvaged bearings from an old set of Rollerblades. I cut one end off and threaded it (M6 die), removed the aluminium tube and fitted the stainless one.
Without the fan running this was the result, with a target temperature of 200Deg C set in RepSnapper.
The heatsink was touch warm, but not hot which was an amazing difference in comparison to when the aluminium tube was used. With the aluminium tube you wouldn't keep your finger on the heatsink if the fan wasn't running. With the stainless tube and the fan running on the heatsink the max temperature I recorded was 178Deg C (readings from RepSnapper).
Note: I don't have any special temperature reading equipment. All the readings were taken from RepSnapper and are for relative comparison rather than specific calibrated temperature feedback purposes.
Conclusion: This was a satisfactory outcome, indicating the potential of a stainless shaft to be more effective as a structural connector between a cold-end mounted heatsink and the heater block (hot-end).
Deduction: This approach to extruder design would eliminate the need for more complex supporting side rods or PEEK T-bars. I will still entertain the idea of a PTFE liner inside the stainless tube.
Observing that some PLA that was on the nozzle had melted completely suggests to me that actual temperature on or near the nozzle may have been much hotter than my by now loosely constructed resister/thermistor arrangement was telling RepSnapper!
Photo showing melted PLA on the nozzle, after RepSnapper showed a reading of 190Deg C, an uncalibrated reading.
Thanks for viewing!