This is a great idea, with good execution. I have completed a full campaign with him and I think he's a bit strict. He should be given 50 rings at the start of the level (ala Doomsday Zone / Egg Reverie Zone) to give players more leeway especially on longer levels.
You're given the tools to play perfectly, but you can't just expect people to play perfectly. A good example of having to play perfectly is Metal Sonic and Egg Colliseum, where you directly are not in control of how long the boss takes... or rather you're forced to wait to be able to hit the boss... in a mod that punishes you for waiting, both of these bosses leave you with very little rings to spare. I feel this is the hardest character mod ive played, not the hardest character mod, but the ring drain does make this mod quite challenging. Challenge is a good thing, I just think it should be less strict.
I also think that hitting a checkpoint should give you some rings. Probably something like 10.
I was hoping that he would be able to collect emerald tokens to get a super form (Hyper Sonic), but he can't. Special Stages are a good way to get lives and continues, so I think such an important part of normal SRB2 shouldn't be left out.
Hyper should have normal ring drain, (so that you would actually WANT to transform into him), his screen nuke would be good for getting rings, and his airdodge would help with platforming. (Alot of times, one mistake as Super Sonic can spell out your doom, so this would be very helpful.)
I dunno if you plan on adding him but if you were these were my ideas.
Bosses in general feel a bit odd how you just walk into them to deal damage to them, especially with Brak how you just walk into him and then stay inside him to defeat him the quickest. It makes sense because you're invincible and a hitbox at the same time, but I have an idea for how this could be solved.
I was thinking Super Sonic could have a Dash ability, like in Sonic Advance's super sonic fights. He will take damage while not in the Dash state, but getting hit will only take away 5 rings, just like in NiGHTS. The Dash ability would function how it did in Sonic Advance, but it can be compared to the Drill mechanic seen in the special stage, as it allows you to deal damage. I think this mechanic could POSSIBLY make playing as him more fun.