The slipper tanks were definately used in combat and were droppable. The most common was the 30 gallon slipper. Here are two images of Spit XIIs with the 30 gallon slipper, which was standard for them. It made up for the higher fuel consumption of the Griffon III/IV as the XII had the same 85 gallons internal fuel as the Spit V.
First image shows the tank in place underneath.
Second image shows the belly of the same XII without the tank in place. If you look towards the rear of the wing under the belly you can see the two stops in place for the drop tank. When the tank was released it slid back along the belly and the rear edge caught on the stops. The airflow did the rest, tipping the front of the tank down and away from the bottom of the Spit.
Next is an example of a 30 gallon slipper tank on a RAAF 79 Squadron Spitfire VIII. Kind of small but definately visible. Again, it was a standard loadout which is why I think it's great AH included it.
There was a 45 gallon slipper tank, but that one doesn't show up much as I think it was replaced by the 50 gallon torpedo tank seen on the Spit IX below. This is the bird the AH Spit IX is skinned as. I've only ever seen these tanks on Spit IXs. Not sure why.
And of course the 90 gallon slipper tank first used on the Malta Spit Vc's that went off the carriers. They weren't used as a combat load out at that point but were used late in 44 when Spit IXs and even the XIIs escorted RAF bombers on daylight raids. Image shows Jimmy Ware of 41 Squadron on the wing of his XII with the 90 gallon tank visible behind him as well as stacks of them in the background.
This image shows an August 44 91 Squadron Spitfire FIX with a 90 gallon tank in place for an escort mission.
I hope that clarifies things on Spit drop tanks. I'm glad HTC included the 30 gallon tank. And I don't think they needed to add the others as it would be overkill for the MA and not really needed for ToD as the 30 gallon would have been the standard tank anyway.
