As per the title.

I am setting up a few older benching systems, and want to maximise drive speed. On an X38 motherboard (specifically Maximus Extreme), is it possible to boot from a PCI-e SSD AIC? I don't want to waste my money if they only work on later boards, or worse still, boards that already have an M.2 slot. I can always stick a SATA SSD in, but if I can save a few seconds booting and running benches that access the disk (without paying silly money), then I want that

I guess it's possible.

I've used to boot M.2 PCI-e SSD with x4 adapter even on s939 with no problems at all. Some LGA775 VIA chipset boards counld't do that (4CoreDualSATA2 for example), like other 95% of them should boot it with no issues.

I think you will need usb bootloader to load the ssd from pcie.

Check "Clover" bootloader