Well, now I own SD3 for two days and I can answer the questions above myself. Yes, it takes a couple of seconds (more) to load a huge 4 GB preset ("Djent style" for example) from the external SSD, the internal SSD's reading speed is about 4x faster. But it's nothing that really annoys me much. And most of the factory presets make use of all parts of the library, so yes, they sound different with only the basic library installed.
And Henrik was right, I did not regret the purchase of the crossgrade. The Grid Editor is just amazing. Finally I don't have to struggle with Logic's piano roll and step editor anymore. Articulations can easily be changed, velocity too. Everything appears to be really well thought-out und the UI is much more user-friendly and clear than I expected. Haven't really got my head around the mixer yet, but I'm sure I will get the hang of it. I get a little intimidated though when clicking through the presets and looking at how much was done in the mixer to get the sound. Holy crap, so many effects, so many stacked instruments, so many different routings... I guess I would never be able to get anywhere near this by myself. So yeah, I guess pre-processed samples and/or professional presets will still be my way to go. Thankfully SD3 has loads of great presets.
And again I have been overwhelmed by GAS and ordered The Rock Foundry SDX, too. That 'tallica Black Album preset I heard in a couple of youtube clips... 