At start of this May I changed Adsense placement in my blog in order to get higher click through rate. So far this seems is working, CTR has raised. 

Now I'm willing to increase Cost Per Click. 

For last 10 days I stacked with 2 image only ad units and one text only ad unit. I believe due to fact, that my blog traffic is pretty modest, I started to receive smaller CPC, while getting higher CTR.

For now I'm switching back to Google Adsense default settings (text/image)

It's still plenty of time to test, what works better and what not. In future I'm planing to decrease ad unit count to 2 or even just 1.