Let's say you have Repeat Spell, Enlarge Spell, Maximize Spell, Empower Spell, and Persistent Spell and are casting from a class 15 node. We will use node classes as "metamagic currency."
You may apply metamagics to spells up to 20 from circle magic. You Maximize, Empower, Repeat Spell, Enlarge Spell, Persist and Heighten a spell (just for an example). Maximize, Empower, Repeat, Enlarge, and Persistent spell cost, combined, 3 + 2 + 3 + 1 + 6 = 15, so you use your Metanode spell feat to reduce this to a level adjustment of zero, and you still have the spell heightened to 20th level....
Granted, that's a lot of feats and makes some assumptions about how the abilities work together....