I think Ashura works well as he is.
The Firedash as base ability gives him a lot more navigational options than base Sonic (Sonic needs better play to keep a fire shield on). This means Ashura can easily access wall blocked paths and shortcuts with less effort.
You can rush down through the stages with him at the cost of being less effective than Sonic at the bosses.
Amy and Shadow cover respectively the Hard mode and the Easy mode, so yeah, it makes sense for Shadow to feel easy.