The 400mHz DStwo is known to be somewhat hamstrung by the speed of data going from a cartridge to the DS system.
Now, I could be wrong on this, but word is the 3DS has an even slower data transmission speed and would thus greatly limit the kind of graphics/data that can be pushed to the 3ds screen. Probably not worth it for the kind of things a 1Ghz chip would be expected to do by homebrew enthusiasts.