I figure this will have to be pretty carefully handled to not be just free unlimited echoes, but Marisa's magic is based on painstaking research more than natural talent, So here's my proposal:

The immediate effect is that improved technique will mean she'll waste less energy when using her powers, so they'll start to be more powerful and less tiring. Over the long term, she'll be able to start researching and mastering spells and abilities from her past life. Rather than just giving her old abilities willy nilly, though, I'll take opportunities where she COULD have gotten an echo, and instead say "She mastered a new spell!" so that she's not getting her abilities the same way as everyone else, but she's still getting them at more or less the same rate.

Does that sound fair?