Do you get preloaded drivers + OS on System 76 laptops?

Yes, it is the same version from the Ubuntu website. They attempt to use components that are fully supported out of the box. They do, however have a single .deb (second link from bottom is the newest), which includes all drivers for all their PCs. This is preloaded on all PCs, and provides drivers for things such as laptop keys, wireless cards, etc. There is no bloatware or other software installed on their machines. More information can be found on their website.

is the x64 Ubuntu 12.04 the exact same one from the website?

Yes.

do you get a tweaked, driver preloaded OS?

No, but you do get better supported hardware that is known to work or give the least of troubles under Ubuntu.