R9 390 will struggle on 1440p only if you push always to max for the new AAA games.
Not from VRAM point but already from brute performance.
If you can lower the settings, R9 390 is quite nice for 1440p.
144Hz is very nice despite of that you can not use FreeSync above 90Hz.
If you can run above 90 fps on 144Hz monitor, any Sync is not really needed in my opinion.
On 2560x1080, R9 390 will perform better compared to on 2560x1440.
Some new AAA games can still push R9 390 to struggle on that, if you max everything.
The thing is there no 144Hz 2560x1080 IPS monitor on the market.
2560x1080 = 2764800 pixels
2560x1440 = 3686400 pixels
1920x1080 = 2073600 pixels <-- the best for R9 390
2560x1080 has about 30% more pixels than 1920x1080.
2560x1440 has about 70% more pixels than 1920x1080
Let say if you have now 60fps (not due to VSync) on 1920x1080, you get about 40 fps on 2560x1080 and on 2560x1440 only ....well...you do the math yourself.
Any fps below 40 makes me feel somehow dizzy and wanna puke after some minutes.