On 1/8th on road....yes, fuel gun on a average person is still FASTER then using bottle...if used by PRO, even better. Using gun on 1/8th is almost like the ONLY choice, that couple seconds faster DOES matter if you are catching up...(personally experience 2 days ago and that several seconds faster pit stop over a 1/2 hour mains DOES make a hugh different).
On Touring car...yeah, I would say depends on the car's fuel tank, the body, the vent/refill hole size, location and the pit person....it required some practice and design of the refuel opening...I would still think using the gun is faster, but probably not as obviously as on 1/8th.
Also, bottle's tube size DOES matter too...I know there are some bigger diameter fuel bottle can deliver more fuel to the tank (bigger pipe deliver more fuel..) however be prepare to get fuel all over if not handling correctly and also my personal experience (way too many)...fuel over receiver = ran away car or DNF....
