Imo this is the same situation as with the PS3. The device can be hacked on <4.5 firmware versions but not above because the exploit has been fixed. And there won't be a CFW or flashcard for later FW versions for awhile or perhaps never. Gateway and those cheap clone makers promising working on later FW versions is just marketing. Imagine they tell you anything above 4.5 will never work.
IIRC, Gateway didn't even find the 4.1-4.5 exploit, so I'd be hard-pressed to believe they'll find a 5.x-6.x exploit either anyway. At least with the PS3, I can downgrade my firmware without a NAND backup of an older firmware, though. That seems unlikely as far as the 3DS is concerned.
I wonder, though... is anyone even looking for an exploit for 5.x-6.x? Those that successfully hacked their older firmwares seem content with just documenting everything for now. I remember talks of other exploits that might still work back when the 4.1-4.5 exploit was patched, but nothing more was ever said about it.