I recommend DXTC DDS (which, in a zip file, are approximately the same size as JPEG) for weapons and most textures, and RGB DDS (which are huge) for landscapes. Unfortunately, Aorta doesn't save uncompressed (RGB) DDS yet, and its compressed DDS quality isn't as high as nvidia's tools.
I'm downloading Alpha 2 as we speak, so I'll have a look at the textures and see what I can do. My build here has the smoother-loading progress bar, so that should make it easier to see what's taking so long :)
Why did you choose .sit? :(