Cursed Seal jutsu, standard sage mode, and Jugo's clan ability are different.
Curse Seal is based off of Jugo's clan ability. In exchange for not causing madness, the seal takes the user's chakra in exchange for senjutsu.
This makes sense, since even in regular sage mode, you're mixing natural energy with your own chakra; so even if every drop of chakra in your body was senjutsu, eventually you could run out of chakra and be forced to leave sage mode (and probably the living world xD)
Regular sage mode takes in natural energy and makes a balance out of it with some of your energies to make senjutsu.
Then, ya got Jugo's clan. Running out of stamina is not something we've seen Jugo do yet, but it's probably quite possible; we can only imagine what the guy ate when he was out in the wild on self-caused exile. It takes in natural energy and almost makes it, like, a third energy for their body, allowing them to transform and stuff.
Summary: Cursed seal might could be considered an implant, but then that would mean that curse seals could be made of every KG, if you have the enzymes that cause the KG.
So... Ideas...?