If I'm reading your question right, the answer is that PCI-E slots have a much higher bandwidth than current SATA III ports, so the speed potential is much faster (PCI-E 3.0 is around 900+MB/s, SATA III is about 600MB/s, from what I understand).
If I'm reading your question right, the answer is that PCI-E slots have a much higher bandwidth than current SATA III ports, so the speed potential is much faster (PCI-E 3.0 is around 900+MB/s, SATA III is about 600MB/s, from what I understand).