Using Shader Model 5.0 with Surface Shaders?

I'm integrating a pretty heavy shader with Marmoset Skyshop and have run out of texture interpolators.

I am running DX11, and am using #pragma target 5.0,

My understanding is that I should have access to 32 interpolated registers (info from wikipedia), however I am receiving the dreaded too many interpolators error while attempting to use 10. Is there a way to get access to the rest of them? I only need to target DX11 devices.