I haven't heard much about the new LS Live, but I own a LS Pro and can tell you it's pretty quick. On my gigabit network, I peak at around 175 mb/s (that's roughly 21 MB/s) download, with an average of about 160 mb. Uploads are slightly less, with an average of about 125 mb/s.
I did hear that the older LS Lives, which were basically a LS Pro with a different firmware, were much slower for some reason. Again, haven't heard much about the new ones, although from just looking them up, they do look nice (and affordable!).
When I was buying my NAS, I found SmallNetBuilder to be a great resource. Be sure to check out ther NAS performance charts. Hopefully they'll review the new LS Live soon and test it.