is pci ssd bootable? curious
Depends on the SSD. Some are bootable and some are not.
is 1x pci big improvement over sata?
Depends on the exact system, but in most cases no. PCIe x1 is 250, 500 or 1000 MB/sec for version 1.0, 2.0 and 3.0 respectively. SATA is 150, 300 or 600 MB/sec for version 1, 2 and 3 respectively. The common case in modern desktops is PCIe 2.0 x1 slots with at least some SATA 3 ports, so SATA is faster in raw transfer. This of course assumes that the SSD is limited by the above speeds and is also ignoring latency which would be better on PCIe.
could ssd be engineered bootable cheaply on a 16x bus?
Depends on your definition of cheap. Sure it could be done, and has been. Most PCIe SSDs are just a bunch of normal SATA SSDs with a normal HBA chip all on one card. Effectively no different than a bunch of SSDs attached to a RAID card in RAID 0. There are some that are just flash controllers on the PCIe bus but they are less common.
how would mapped directly into the virtual address space change things?
That would change how the system "sees" the SSD dramatically. It would look like RAM except that it would not lose it's contents when the power was shut off . The CPU could read or write to it directly instead of through a controller. Software would likely make it appear as a normal drive to the OS and users.