It may need to be tweaked, but probably does not. It doesn't change anything about the video card -- that's why it works in a wide variety of different macs using the same code. The problems mostly occur with iBooks that have video cards that don't support spanning at a basic level, I think.
Once it does get working, it should actually be better, since that's a substantially faster and more capable video card than the one in the previous iBooks. Although, I guess you could also argue that, having more capability, more stuff will be offloaded to that video card than the old one, meaning more video ram will be used up, meaning, that, for the typical usage profile with the new card, you might see more deterioration in performance with the second monitor plugged in than you see on the old iBooks. But it'll still be faster than the old iBooks.
Anyway, though, I would wait and see if I were you, too, though.
I have a new 14" and have the hack installed. It works with my 17" 1280x1024 LCD. The only bug I found was my clock resetting on each reboot. I zapped the pram and retried the hack and I no longer have the problem. I think the clock reset may have to do with having the video adapter plugged into the iBook without the second monitor connected.
I havent had any major slowdown effects except for when I am dragging from screen to screen. The edges of the application window have a jagged effect, but if I am just moving it within my secondary monitor, it is as smooth as on my iBook. Importantly, Expose and Dashboard run smooth. You dont have anything to lose. All those spazzes who scare people off boards are usually misinformed with older iBook (Rage card) stories or freak incidents.