> 8x PCIe v2 lanes = 16x PCIe v1 lanes. So it's the same bandwidth, right?

Right, but while in theory there shouldn't be compatibility problems, some PCIe v3 cards have been reported to not work in PCIe v1 slots.

> the PCIe 1 slot [...] only has four dedicated lanes

Yes, the x16 slot is connected by only four lanes, as was announced last year.

> and four switched lanes, shared with the other PCIe slots
> (i.e. the 16x, 4x, 1x, 1x, and "Xorro" slots)?

According to the block diagram on page 8, the four switched lanes have nothing to do with the x16 slot, but are shared via PCIe-PCIe switch between x4, x1, x1 and Xorro slots as well as PCIe-PCI switch (for two PCI slots).