The limit is 200000 particles/ sec, I believe, but quite frankly I never exhausted it. If you were, rendering would be terribly slow. I think you need to rethink your approach to the problem. It seems like your biggest problem is to control your particles and their density and no matter how you tweak your setup, it will never work this way, I grant you that. What you need is some custom particle that resembles multiple dust/ sand grains and you also need to work with multiple layers of particular. This way you can have one emitter to fill your type and the others contribute to the swirling in your scene. You can easily ensure your text becoming solid by e.g. animating the contrast of a Fractal Noise effect used for your custom particle.