In all fairness, those curse marks were both used on characters with really strong kekkei genkai and part 1 naruto was able to almost tie with curse mark Sasuke. Either way it has potential for a good villain arc.
Part 1 Naruto...One-tailed Cloak Naruto. I'm not
too sure how strong that is.
You can match Curse Mark Stage 2 to a Version 1 One-tailed Cloak, but how strong that cloak is compared to others is a bit difficult.
Version 2 Four-Tails Cloak is about as strong as a Sanin. Fours tails managed to almost kill base Jiraiya, but a
much stronger Naruto used it against Orochimaru and he was only like...on par. That's against an Orochimaru who couldn't use his jutsu properly after Hiruzen damaged his soul in Part 1.
I agree that it would probably boost you to maybe Chunin level or so...but that's as specific as I can get with what I can recall.
While I do see what you are saying, that is still hard to abuse. It took ages for Jiraiya to summon the elder frogs to fuse with. In a fight not many people would be able to survive without using their hands for as long as Jiraiya was able to. That's why he barely ever used it, even though it was very useful. Perfect sage mode just makes you too powerful, although the time limit is a nice touch to make it actually fair.
You can't move at all when becoming a perfect sage though. So the restrictions for that are even higher. The only way to transform into a perfect Sage as fast as Naruto or Minato does is to do just that, be either Naruto or Minato, more precisely, you need to be
both a
Perfect Sage & a
Perfect Jinchuriki, then the time limit for Sage Mode can be reduced via the Tailed Beast's help.
Jiraiya's mode is still way easier since he can still move while doing it, unlike the perfect motionlessness needed for Naruto's Sage Mode. (Prior to him becoming a Perfect Jinchuriki.)
The Shadow Clone method is also somewhat feasible...but not really. Naruto's kind of the god of shadow clones. It's explained a few times that he's so familiar with the technique that his clones can do things other's cant, e.g. using them to train Rasenshuriken.
Kakashi specifically says it's something that could only be used by someone as skilled with shadow clones as Naruto. Anyone else would've collapsed from the sheer number of clones and exhaustion that would be transferred back to them.
And even someone like Naruto can only create 5 shadow clones total while charging Sage Chakra...that's from like 2,000 down to 5, it's a pretty heavy nerf on its own.
It gives a limit to using the Mangekyou so that it's not as overpowered and has a cost for using it.
Hmmmm. It'll be just as powerful, it only limits is usage. Well, I'll see what the other opinions are.