Well, could be done technically I guess, but doesn't make much sense performance wise, I'd say
You'd simply end up with a plain 7/16" (11.1mm) steel tube inside the much wider, 16mm (outer diameter) glass heater cover with the 14mm M taper then... That's more mass to heat up while at the same time reducing the airflow significantly. After all, the 7/16" steel tube has an inner diameter of only 9.5mm, while the 16mm glass tube sports an inner diameter of ~12mm there.
One thing that makes the GTR the GTR, is its use of the wider glass heater covers with the 14mm and 19mm male & female ground glass tapers, while the HI Log goes with the plain 3/8" (9.5mm) & 7/16" (11.1mm) steel tubes for heater covers (plus the 10mm & 12mm plain glass tubes, for the HI glass core versions).
And while @Alan
referred to the 7/16" steel core as the 'most powerful among the HI heater covers', I can guarantee you, that all of the 4 tapered male/female joint versions of the GTR glass heater covers are quite beasts
in their very own class, taking the HI experience to a completely new level
After all, I've spent a week building my own DIY versions of all 4 GTR glass heater covers along Alan's specifications (with only some minor modifications) and had ample time, testing them all out!
They're magnificently 'brutal'... for lack of better words! And can also roast significantly larger loads, than a HI with the 7/16" steel core could
If you want even more power for the 14mm M GTR, simply wait till Alan has found the right diameter steel tubing, to cut the 14mm male taper on