With a DD hubmotor, yes.
A geared hubmotor does not connect to the outer shell.
So while it would indeed have better heat transfer from stator into magnets, the magnets then have nowhere to shed their heat to, except into the insulating air inside the outer shell.
Eventually that heat will pass thru the air and the shell...but it is nothing like the ability of a DD hubmotor to do this, where the magnets are directly connected to the outer shell, and thus can shed their heat into the air, where a heatsink will help it do so by giving more surface area to convect and radiate that heat from.
Basically, as far as heat shedding goes, a geared hub is like wrapping a thin blanket around a DD hub.
The oil or ATF that people have used in geared hubs helps by directly carrying the heat away, as it is much less of an insulator than air.
FWIW, I couldn't find anything on Grin's site called a "TIM and radiator assembly". The closest thing I can find is the hubsinks, here:
https://www.ebikes.ca/shop/electric-bicycle-parts/motor-hardware/cooling-mods/hubsink-set.html
which are for Direct Drive motors:
Quick Overview
V2 HubSink Set for Optimal Cooling of Direct Drive Hub Motors with Statorade. Includes thermal grease and hardware.
V2 HubSink cooling fins for greatly increasing the cooling capabilities of 205mm (9C, Crystalyte 'H', MXUS etc.) direct drive hub motor that contain Statorade. The 6-piece solution bolts onto the ring of existing direct drive hub motors and provides much more area moving heat from the magnet ring to ambient air, increasing the cooling effect by up to 20% over just Statorade alone.
Note The new Crysatlyte 'UFO' motors are slightly too small in diameter for the Hubsinks to fit tight. Usage with a UFO motor requires a thermal pad material to make up the difference in diameter.
ON the statorade page
https://www.ebikes.ca/product-info/statorade.html
it has this in the FAQ:
Frequently Asked Questions:
Will this work in geared hub motors or mid-drive motor systems?
No, not really. Geared hub motors do not have their rotor shell exposed to ambient air, so increasing the heat conductivity from the motor stator to the rotor does not help get the heat to ambient air outside. Similarly, most mid-drive motors (like the Bafang BBSXX) are inrunners, where the magnets and rotor are on the inside and the heat generating stator, and conducting heat to them would serve no purpose. The benefits of Statorade are really only present for outrunner motors (magnets on the outside) where the rotor is exposed to air flow.
