That guy is an idiot, he probably set shadows to high, but in Lost Planet the difference between high and medium shadows is non-existant (Except for a performance hit). The game is very poorly optimized, we can all agree on that, but I urge you play Lost Planet on your own and see it for yourself, it's not about DX9 or DX10, it's just poor optimization. But Lost Planet is one of those few games where I say that you don't need constant 50fps+ to enjoy it. (Mind you I almost never say that!)
Also, now that Bioshock has been released and plays just as smoothly under DX9 as it does under DX10 with improved effects, I think I can say that it is not "necessary" to wait for next gen cards to play DX10, however what really will be necessary is not to go below the 3 top cards (GTS 320/640, GTX, and 2900XT).