Is it worth it to cf with my mobo

Hello. I have an Asrock 970 extreme 3 r2.0 and currently a single r9 270x. 

I would like to cf dual r9 270x in the near future but I checked my mobo and it says it has 2x pcie2.0 x16 (1 x pcie2 @ x16 mode, 1 x pcie4 @ x4 mode) I've read that running x16/x4 cf would not draw out the full potential of the second gpu running in x4 mode, my question is, by how much? how much performance would I be missing if i cf these 2 cards. Are we talking about huge performance loss or really minimal?

Should I get a new motherboard instead of a new gpu? Common advice I've read is to sell current gpu and get a 1 single more powerful one, my problem is in my area I doubt I'd have an easy time selling my r9 270x that's why I wanted to cf.

Thanks in advance for the help.

Option 1, sell video card and get an upgrade, Option 2 new motherboard. How much FPS drop, Answer - hard to tell as is it depends what resolution. But roughly across the board 5.5% performance difference