If you're talking about just flashing the BIOS to make it look like the card is a 290X, that's useless. The 290X has the full Hawaii GPU active, whereas parts of the 290 are shut down.
Some of the times, the parts shut down don't work anyway, so it's useless to re-enable them. It could even hurt your card. On the other hand, sometimes they're just fine and were shut off for rebranding purposes.
If you're talking about enabling the parts of the GPU disabled on the R90 to make physically identical to an R90X, that's a different story. I haven't heard of this; it could very well be possible, but keep in mind that this obviously violates your card's warranty, and could damage it if something goes wrong.