If you are going to give this a spellcasting progression, then you should make the ability to cast spells (arcane or divine) at a certain level one of the prerequisites. I don't see this as a spellcasting class either. I'd recommend removing the portion where you say add +1 level of existing class to the chart. The mime can't cast any spells that have a verbal component unless he or she has the Silent Spell feat. However, bard spells cannot be enhanced using Silent Spell feat.
The prerequisite of Perform should really be Act (Mime) as that is a standard skill.