Crossfire on a 4x and 16x board?

Just out of curiosity (and the fact that I have a little unexpected spending money), I was considering getting a second 6850 to crossfire on my 970a-UD3. This would put the second card in a pci-e 4x slot, which I know would bottleneck the card by about 10-15 percent. I'm wondering if the performance gain would be worth it, but I'm unable to find any articles that give actual benchmark/framerate results. What are your thoughts? Good idea? Stupid?

10/15%?!? It's more ligne 1 or 2%...

Don't worry!
You'll have a 580 perf, and with a 20% OC, a 7970 like gaming machine.

Edit: the rig in my sig gives me 6550 pts in 3dmark11 with 7400 graphic points, 85fps in dirt3@1080/ultra/8xAA and 45fps in Crysis WH@1080p/enthusiast/max AA...

Really? So have you had any issues with microstuttering or any problems running certain games? This would be my first dual gpu setup.

A while back I looked into this matter for my motherboard and found this :
http://www.legitreviews.com/article/1500/9/

My board does 8x/4x when crossfired and performance, while not as fast as 8x/8x or higher, keeps up with those boards. Anyway, you will gain more speed regardless, going crossfire.

Not really bothered with micro stuttering until now. I didn't notice any actually.
Bad experience was with rage: it's worse in xFire than with a single card, but Dirt2/3, BF3, COD MW3, The Witcher 2, SeriousSam3, NFS (all), f1 2011, Crysis 1/2, The Darknes 2... and I might forget some, run very well in xfire.

It's a link to my 3dmark11 results:
http://3dmark.com/3dm11/2729853

You only really lose out on bandwidth which is minimal these days with newer cards.

There won't be much of a performance loss on a 6850 in that configuration. I've done plenty of research on it because I was considering crossfiring myself.