If we're talking brightness, I for one would take "a bit" of a hit in Lumens for the tail switch functionality, especially given how the brighter the output, the less relative brightness your eyes perceive :)
I would say a good quality switch shouldn't limit the current to such extent to noticeably decrease brightness output and to have a considerable impact on the runtime due to the energy losses in the contacts.
I guess I would take that in consideration for a monster flooder with multiple powerful emitters though and even then, I would still consider the tail switch functionality benefits regardless.
I'm looking at the Sofirn C8F with the bypassed springs and the triple XP-L HD emitters for ex. - it takes quite a lot of Amps through the tail switch and it's still perfectly fine with it.
I would compare the C8F's triple XP-L setup with an SST-40 and even a tad more than that. In fact I wouldn't probably even live without that tail switch on the C8F - Hard to un-see it after you've once seen it :))
But, then again, I guess that's just my preference, I'd take more features/options over sheer power and efficiency. It's not like I don't love high efficacy wherever possible, but, I guess.. slightly different priorities.
EDIT:
Ah, and I forgot to mention that like the added length of the grip area with the added tail switch - the standard Lumintop body always felt too short and a bit too thin for my hands.. :|
Also I like the extra width of the tail cap of the Convoy C8+, it just adds another layer of comfort to the grip, it just feels a bit more secure in the hand, overall, it just feels natural to my hand.
I guess a 21700 would be a nearly perfect size for me at least. Still wouldn't drop the tail switch over brightness and a bit of losses, even with the 21700 body regardless, given the choice.
Oh but if the tail switch would be an e-Switch, then I'd rather drop it. That would be just at the edge, under my threshold of Pros VS Cons a tail switch would add to the full package.