FWIW, PCI-e has never had backwards compatibility with PCI. They're two vastly different protocols that were somewhat standardized by the same standards body. So, PCI-e has always been vastly more performant than PCI since 2004.
They're not very relevant nowadays, but PLX and others make bidirectional PCI-PCIe bridges that I think were mostly used so that companies could ship transitional products— eg, do a new board layout that accommodates the PCIe slot/power/whatever, but add the bridge IC to it so that you don't have respin your super fancy custom ASIC.
