An SPTextField will always break batching - even if you use a bitmap font, and even if that font is in an atlas with all of your other textures. So you will always see a 1 to 2 draw call increase per text field.
I played with this a year or so ago. I created my own text field class that used SPBitmapFont but didn't create it's own quad batch, and hence didn't break batching.
Ultimately I found that it was pointless, as yes my draw call count fell to almost zero, but performance was completely unaffected, as the vertex overhead of a text field (often comprised of many quads) outweighed the draw call overhead.