pex 8647 pci-e switch

Hi!

If only one (not two) PCI-E x16 graphic card is connected on PEX 8647, does that card use all 16 lines of PCI-E bandwidth?
I’m interested about SuperMicro SuperServer 2026GT-TRF. It has PEX 8647 for two cards, and I’m afraid that there will be big bandwidth difference among cards.
Is it better idea to put just three Tesla cards because of bandwidth (two directly on chipset and one on PEX 8647)?

Please, help me.