For sure do not expect huge difference comparing 290X to RX480 in matter of performance.
I don't know how 290X scores nowadays with new drivers but i can reach 16000 graphic points in Firestrike, 6900 graphic points in Firestrike Extreme at 1480/2100. You can treat that as a reference and compare it to your overclocked 290X. There are some golden chips on the market which can reach 1560 MHz with reasonable voltage as well.
I have water cooled RX480 and i can reach 1480 MHz @ 1.3V on core, if i will figure out how to properly cool down VRM i will be able to use it 24/7. Power efficiency of RX480 on 1.3V isn't perfect as the card needs 225+ W to run on such clocks. comparing to 295-350 W on 290X. For sure it is a huge improvement of AMD but it's not a huge difference.
My card on stock voltage is able to achieve 1340 MHz, with +100 mV (1.2V) is able to reach 1420 MHz. As you can see to increase the clocks by 60 MHz up to 1480 MHz i need another +100 mV (1.3V).
If i remember well power consumption on 1420 MHz is around 150-175W.
If i would be in your position i would stay with 290X as it is a great card and in some games it may be even better than RX. If there will be some increase it will be small. Wait for VEGA.
Regarding cooling, throttling and so on. Forget about overclocking on stock reference cooler. You can boost clocks to 1400 but temp throttling will kill the performance. Most ppl who bought RX480 with Ref cooler undervolt the card to stop throttling.
There are three situation when RX is throttling:
1) due to temperature of Core
2) due to insufficient power budget (card with stock bios is limited to 165W - according to my experience card start to downclock at 1400-1420 and 1.2V)
3) due to VRM temperature over 115 C degrees
Water block on the chip is a good idea, temperature on my chip is below 50 C under load. The biggest challenge is to cool down the VRM section, on ref card VRMs are located in front section of PCB which is very short. It's really hard to find a proper heatsink which will fit into this gap without modifications.