RE: "...so a 4x slot has the same bandwidth of a 2.0 8x slot.."
Any NON GPU slot is running PCIe 2.0 and not PCIe 3.0 -- it would be nice but for now it's as I stated:
Recap:
16 lanes are GPU (PCIe 2.0 or PCIe 3.0) x16/x0 or x8/x8
8 lanes to the chipsets in PCIe 2.0 only ; so a x4 PCIe is 2.0.
Exception is MOBO's with the PLX chipset {handful of high-end Z77's} x16/x0/x16/x0 (x16/x16), x16/x0/x8/x8 (x16/x8/x8), or x8/x8/x8/x8 funneled back to PCIe 3.0 16 lanes IF all sub-components are PCIe 3.0 compliant: Ivy Bridge CPU, PCIe 3.0 GPU (GTX 600 series/HD 7000 series), and a Z77 + PLX MOBO.