Well, I only have Spectrasonics and a few other similar sample libraries - Nothing too massive. So I can't reliably advise you shouldn't use Thunderbolt for your particular libraries, but I would definitely take notice of what Nigel has to say as he is much more experienced.
Yes, as Erik has just pointed out, where Logic lives is important. Personally I don't see the point of moving Logic off your internal SSD - It's where mine lives and the SSD performs superfast internally.