First of all, you don't need position in the G-buffer at all. The position of a pixel can be reconstructed from the depth buffer, knowing the camera setup and the pixel's screen-space xy position. So you can get rid of that whole buffer.
Also, you don't ordinarily need tangent vectors in the G-buffer either. They're only needed for converting normal maps from tangent space, and for parallax mapping; these would be done during the G-buffer fill pass (when you have tangents from the mesh you're rendering), and the G-buffer would only store normals in world or view space.
Material properties like colors, roughness, and metallic are usually just 8-bit values in the G-buffer, since they're sourced from 8-bit textures. Same for AO.
Height is also not needed in the G-buffer unless you're going to be doing some kind of multi-pass blending that depends on it, but if you do need it, 8 bits is probably enough for that too.
Normals can be benefit from being stored as 16-bit values rather than 8-bit. Half-float is okay, but 16-bit fixed-point is even better, as it gives you more uniform precision across all orientations (half-float is more precise near the axes and loses some precision away from them). Moreover, you can cut them from 3 components down to 2 using octahedral mapping.
So, at the end of the day, a minimal G-buffer might look like:
- Material color + metallic: RGBA8
- Octahedral world-space normal + roughness + AO: RGBA16
and that's all! Only 12 bytes per pixel.
Alternatively, you could use an RG16 buffer for the normals, and move roughness + AO into a separate 8-bit buffer. That would give you some room to grow should you eventually need more G-buffer components of either 8-bit or 16-bit sizes.
