Webgl texture atlas performance thoughts

Hi, given that the current webgl specification doesn't support texture arrays, I am implementing some sort of variable texture atlas mechanism in my last engine.

Essentially I pass to the shaders a couple variable multipliers for the texture coords to allow for arbitrary atlases to be used with any mesh.

My main concern is: won't multiplying every texture coord counter the performance gain given by using only a few large textures instead of switching many little ones?

Any thoughts or personal experiences for this scenario?

Thanks,
a.