Here's what I'd estimate:
1. GPU power consumption. Going by the OP's results, the windforce card pulled up to about 80w extra at 1040Hz, maxing out at about 680w. Cranking the card up to 1100Hz would add an extra 10-20w on to that. For 2 windforce cards you could expect 700w power consumption under air cooling.
Going by kitguru's reviews of the windforce and the MSI, the two cards pull almost exactly the same power, HOWEVER the gigabyte comes with a stock clock of 1040MHz, whereas the MSI stock speed is 977MHz. If we assume their power figures were both at stock speed, you can expect the MSI to consume more power when they're running at the same speed. Add on 50W to be safe (up to 750W requirement)
2. CPU power consumption. Now the test system uses an i7 3820. The difference in power consumption between that CPU and an fx-8150 is *about* 30W at medium load (very rough estimate based on 89/184W and 104/229W load/idle figures for a full pc system using the i7 3820 and fx8150 respectively, from guru3d). With a mild overclock, let's round it up another 50W to be safe.
- So in total you should be just fine power supply wise (my conservative estimate adds up to 800W full system useage).
3. Temperature wise, you're going to have issues. Taking into account the motherboard you're using and the case/fans, I see a potential issue in that there is no side fan on the 230T to blow directly between the graphics cards. Check out https://forum-en.msi.com/index.php?topic=180405.0 this guy has a similar set up to what you're considering, and is reaching 94 degrees on the top card at 1085/1350. The MSI cards run hotter than most other r9 290s (non-ref). If you had a case with a side window and 2 fans blowing on the gpus, it'd be a slightly different matter. Either way it's going to be pretty damn loud with all those fans blowing at 100%. It would probably work, and the cards would probably survive, but the VRM temps would probably hit 100+ degrees on the top card, so I wouldn't want to use this setup myself for any extended period of time.
I'd recommend attaching an NZXT G10 + liquid cooler (e.g. corsair h55) on to the top card, and leave the bottom card as it is. That should safely keep both cool and allow for potentially higher clocks. (would set you back another $100 or so)
