I always thought that SNES and GBA graphics are almost same?

The graphics are very similar looking, but the hardware is different as are the capabilities. For one thing, the SNES and GBA have the same color range (65535 colors IIRC) but the SNES can only use a 256-color pallete at any time, a choice from that color range, where the GBA can use them all simultaneously.

Also, GBA emulation is not really possible on the DS without certain allowances. Note that the only GBA games run on the DS not to use extra hardware (google DSTWO flash cart) were actual games run through Slot 2. This is because it runs the actual GBA hardware inside, making the job significantly easier. The DSTWO adds a 500mhz cpu contained in the cart which helps even more. The DS can't really emulate the GBA by itself, as far as I've seen. The hardware isn't up to snuff.

Note that accessing the GBA hardware is (supposedly, I'm not going to stick my neck out here in case someone came up with new stuff) technically incapable of running the GBA hardware form from the DS card slot, which would be more akin to emulation.

That's not technically true, since the GBA screen is limited to 32,768 colors. The Hardware is theoretically capable of addressing more, but the screen isn't, so the GBA is limited to the same overall color pallete as the SNES, with the exception of the pallete limitation.

so.... it's not possible to emulate GBA with cx?

Sure it's possible, but nobody knows what performance it will have until somebody writes one.