I doubt the rumors that the Northern Islands will be literally Twice as fast as the previous generation. Maybe 40-50% faster like the ATI 4K - 5K series.
The HD 5xxx and the SI are almost identical in performance. Except the SI scale much better in CF, and use less energy.
The revolutionary GPU performance milestone won't be until 2013 with Nvidia's "Maxwell" cards, which are said to have roughly 16 times the performance of the current Fermi's - the real Crysis killers!
Imagine running Crysis/Warhead on very high/enthusiast at 2560x1600 with 8x AA, maybe even with graphical mods, and get over 100 FPS!
You are mistaken in looking at the naming scheme as a means to compare the 2 generations.
The HD 68xx series is the new midrange cards. Comparable to last generations HD 57xx series. The upcoming HD 69xx series will be their new high end cards. The HD 6xxx series already seems to have your "40%-50%" increase over the HD 5xxx series.
I'll grant the new naming scheme has thrown most people off since it seems logical to compare the HD 5870 with the HD 6870 but that's an invalid comparison for the cards themselves. You have to look beyond the card name.
It feels like a number naming mix between the HD 2-5xxx series and the 1xxx series. Just glad they didn't bring back XT, XTX, Pro, PE, GTO, XE, GT crap. Now THAT was confusing.