The extra 16GB is what the drivers allow the GPU to borrow from system memory. Exceeding the amount of VRAM available will cause the GPU to pull stuff from system memory over PCIe which is much slower and may cause severe performance degradation depending on how much extra stuff ends up going over the bus. If only rarely used assets end up in system memory, the slow-down from exceeding 8GB may not be too bad. If lots of frequently used stuff lands in system memory, it can turn into a slideshow since 3.0x16 is 20X slower than on-board memory on top of ~200ns higher roundtrip latency.
You can still try it. Worst case, you wasted your time downloading the hi-res texture pack.