I'm not sure SDXC standard have potentials to compete with SSD on PCIe or SATA even when based on the same kinds of NAND chips. Those standards are written by Japanese companies making digital cameras. But I would also like to see full-size SDXC slots on tablets and smartphones instead of microSDXC slots.
BTW, a 128GB SDXC offering 45MB/s max at US$199.99 cannot compete against 256GB SSDs offering 10x faster rates at about the same prices. So it's better to find a way to integrate those SSDs into tablets and smartphones.