How can I get the CTR for a particular ad copy, considering only Search impressions and clicks? The overall CTR for each copy is very low (0.1% - 0.2%), so it's not useful for determining what ad copy works best.
Create a 2nd ad for the same campaign, use different copy. Route the traffic to the same exact landing page and then measure the difference between the two groups.
Once you have established which of the 2 or 3 ads in the ad group is doing the best (say by a margin of at least .5%) then launch one or two new ads that are variants of the one that has the highest CTR.
The best way to do this, imho, is to also track backend conversion...that way, you can optimize the whole set of variables, from beginning to end, with time.