I think at 4k resolution it could realistically be on par with a 295x2, at 1080p probably not.
even at that, it's still better to get a 295x2 or even 2x 290x because of price. a 390x will be £700+ where 2x 290x u can for around £450, 295x2 can be get for around £500-£550
getting 2 390x would mean most people would need to upgrade their case due to lack of radiator space.... and the 390x air cooler version will be hot and loud again i'm guessing