In my view powder coating of rear drums is OK provided that the drums do not
reach temperatures high enough to burn the thermoplastic powder used.
It is probably down to the driver and more importantly how he uses his
brakes at the end of the day. Aggressive braking causes heat, excessive heat
will cause damage to the coating.
Another consideration is the mass of the drum. The larger the mass, the more
heat dissipation. The Sprite, as I remember, has very lightweight drums.
If, after a normal run in the mountains, you can place a hand on the drum
without burning yourself powder coating will be fine. If you burn your
finger tips (do not sue me) but you will have saved a few bucks, powder
coating is not for your drums.
My two pennyworth,
TR4A CT64306 O