If you do the manual disc brake conversion instead of power, is the performance still a big improvement over drums?
That's a trick question, right?
No, disc brakes do not work any better than factory drum brakes. Now, before I hear the screaming start, you must consider the comparison. Are your brakes working poorly? Then anything is better. Are they working right? Then discs are not better, as you can only stop as hard as your traction allows, and drums (working properly) will do that just fine on the street.
I say this as many people are disappointed by discs if their original drums were working well and adjusted properly. So, having said all that, the benefits of discs are reduced brake fade under intense use, reduced susceptibility to deep water and splashed road film, greater selection of pad material, and due to our classics becoming aged—parts availability. So, if you have poor braking, then you can fix what you have and upgrade specific parts while you're at it if you want increased fade/film performance, or use discs if you drive in deep water or snow a lot. And if racing discs can be a better choice. But most other reasons are generally for looks, style, or ego. While I used manual
drums when racing a Stock-class car to stop from high 10s at 130+mph and 4400 pounds; I drive sanely on the street, and I mostly use discs now (still manual because I like the feel of the feedback), and I don't race as much, but want to look like I do.
TL;DR - Drums can stop as well as discs on the street, but use whatever makes you giggle.