Radeon 7970 in MacPro with PCIE 2.0 for FCPX ?!?!?

Good Evening,
In reading a reply of a user who appeared to have knowledge on the subject, it was mentioned that a Radeon 7970 could replace the 5870 in a MacPro for use with FCPX.
I have a "Mid 2010" MacPro 12-Core Westmere w/24GB Ram, 8TB (4x2TB) Storage & a 1GB 5870 Graphics Card. I am looking to upgrade the graphics card on this device to improve render times & anything else GPU controlled/contributed to in FCPX. This MacPro, as with most, has PCIE 2.0. The 7970 is a PCIE 3.0 card. I know 3.0 is backwards compatible & will work in a 2.0 slot at the reduced gigatransfers associated w/2.0, but due to said speed reduction, will I even see a difference with the 7970? I've been researching this for some time & can't seem to find a definitive answer. Please advise...
Thank you in advance for your time & any assistance/advise offered!!

Here is some test results.

It is an OpenCL card, too.

My 2008 MP runs the 5870 really well. Would be nice if it supported this card with OS X 10.8.

Reading Amazon reviews, one guy says Sony Vegas only got a 20% increase in performance. If that's all FCP X would get, its not worth the money.

It also looks like the 7970 only has the option for 1 DVI PORT. I need a card with 2 DVI's, 2 mini display ports & 1 HDMI. It would appear that I can only get the 7870 in that config. Am I correct in this belief or is there a 7970 in that config? & if I may further muddy the waters, should I get the XFX, ASUS, Diamond, HIS, EVGA, etc. version of the card for use with my MacPro & FCPX?

You can get a mini display to DVI adapter easily.

And remember, when you connect two monitors, your apps ability to access the GPU and VRAM is cut in half. It will only access the GPU power and VRAM allotted to the primary monitor.

Smarter to get two cards to run two monitors, keep the speed optimum.

I cannot install two graphics cards because I also have a AJA Kona LHe+ video card installed. This means that I have to be able to get the most out of one card. If the VRAM & GPU are divided by monitor count, then I should still see a difference as my current card only has 1GB (500-512MB per display) and the new card has 3GB (1500-1524MB per display). But again, what about the step down from the 7970's PCIE 3.0 to my MacPro's PCIE 2.0... How significant will the step down be & will the benefits be worth the upgrade in the end?

If the 7970 is that much more powerful and faster than our 5870, then you should see quite an improvement.

BTW, I emailed the author of that Barefeats article I linked to. Here is his response:
"If you wait a few days or weeks until 10.8 3 comes out, there will full support of the 7970 -- and, yes, it will work in your Early 2008 Mac Pro."

Thx!! That's great to know!! I'm still on 10.7.8 & have waited purposefully on a "stable 10.8.?" prior to upgrading. That has appeared to be the case for a while, but this card issue has given me pause.
Does you or the author have any insight on the PCIE step down issue? There is a 2GB 6870 that is PCIE 2.0 for considerably less than the PCIE 3.0 7970. If I'll only see a "size" difference then I can get the 6870, but if there are architectural/processing/power/speed/etc. benefits as well, than the additional investment for the 7970 would be well warranted.
Please advise...

Thank you for all your replies thus far!!!

That should have read "Do you or the author...". Please forgive my improper English!!

From the posted BareFeats article, it looks like quite a significant power boost. Remember, they're testing PCIe 2.0 Mac Pros, too. Also remember the 7980 won't be compatible until OS X 10.8.3 is released.

I personally will be waiting until then to see the test results. I'm sure Barefeats will publish them right away. Then I'll take the plunge.

Another issue, it seems like a card that will migrate to a new Mac Pro when/if Apple releases one later this year.

Here's some very promising news!
Sapphire Previews Radeon HD 7950 Mac Edition Graphics Card for Mac Pro