But where does the extra haki come from?
Tengu explicitly says that Enma takes
your haki.
You cannot have 200 percent of haki. We know that haki is a finite source. Lulffy cannot use haki for ten minutes when he uses G4. Lulffy states that haki runs out as you use it when fighting against Dogtooth
The difference between Lulffy's G4 and the stunt with Zoro's arm is that Lulffy actively pours excessive amounts of haki in G4 and willingly, whilst Enma took a huge amount of Zoro's haki unwillingly at once.
What you are saying could have happened if haki was a limitless source, which it isn't as we already know, and if Enma could draw haki out of anything, which we also know is false as it draws your own haki. Enma cannot draw more haki than you have, as it is literally impossible to do so.
Enma cannot draw out 150 percent of haki when you only have 100 percent.
Even if we were to go with this premise how would Zoro fight then if only Enma can draw on that amount? It would mean that the slash with Enma would devour the other two slashes, akin to Shusui back in TB.
If only Enma can draw more haki how did Oden wound Kaido with both his swords? Do you think he was using like 100 percent with Ame and 150 with Enma? Then the cut with Enma would have been more devastating.
Zoro was cutting bamboo trees in the same chapter that this happened and his body wasn't shrinking. I believe it was the surprise factor and that Enma drew on vast amounts of haki unwillingly that made Zoro's arm shrivel.
The point is that Enma cannot take more than you have. And you cannot have more than 100 percent. Since haki is a finite source, you cannot just tap into some hidden haki. Your haki reserves can grow as you become stronger, but there's still a limit.
Mastering Enma gives Zoro better haki control, better efficiency, it probably boost Zoro's overall COA, allowing for better offense and better defense.
Of course I could be wrong, but I am basing this on what we have so far.
If I understood properly what you were saying is that it will improve his efficiency, which I agree with. As long as we agree that it's Zoro's own power, it's good.