if u already got cf mobo so y not give it a try. its worth it. 4770 gives very good scalling.
im on 4770 cf on a blood iron p45 mobo and it run at x8 x8. afaik the performance between 16x 16x and x8 x8 different is around 2-3fps. so not much different unless on hi reso maybe.
the power consumption is lower a bit then 5850 or 4890.

i got myself a 13k gpu score on vantage both on stock. for crysis on fullhd high setting max fps 45 average 35.

REVIEW see games review. it does give a very good scalling on farcry2 and hawx. other games too it stands out among hi end card.

for me i would suggest u to try it since u already have a cf board and single 4770. get 1 more pcs n rock on