My experience doesn't match with what your garage say. Not by much, but my T32Gt have lasted consistently longer then my T31GTs did. Small margins, I admit, but every little helps. 5000miles is the norm for a rear tyre for me. Hills, two up touring, textured roads, weight etc.
My current set are promising to last longer.
I note the Bridgestone T33s have been announced with sizes that match the ST1300. They claim 47% increase in life. But no mention of GT version yet, but it does say the construction is now such that it is suitable for light and heavy bikes.
Bridgestone Corporation (Bridgestone) today announced that the launch of the BATTLAX SPORT TOURING T33, a new sport touring radial motorcycle tyre, is scheduled for January 2025.The BATTLAX SPORT TOURING T33 is a next-generation sport touring tyre that maintains the necessary touring performance...
press.bridgestone-emea.com