Pokemon-go – How effective are Razz Berries, quantitatively

No, the razz berries are not causing this. Razz berries do not affect your throw at all; they only make the Pokemon more likely to stay in the ball once you get them in there. Although it says it affects your next throw, you can tell it actually means your next "catch" because you cannot use another razz berry until you get the Pokemon into the ball.

It sounds like you may be having the same problem as described in the question Ball curves on straight throw, or perhaps you are throwing curve balls on accident, or you are just not throwing it as straight as you think you are.

There are a number of people that insist using a razz berry does curve the ball. I personally believe that this is a combination of coincidence and confirmation bias. People use razz berries on harder Pokemon, which tend to be farther away, which makes it more noticeable when a ball doesn't go where you expect it to. Once you start to believe that, you also tend to notice when the ball curves when using a razz berry and dismiss it when it curves without a razz berry.

I initially had this same thought when I first got access to razz berries, but some informal testing convinced me that it's not true. My personal experience has been that the ball is equally likely to curve on a far Pokemon whether I use a razz berry or not, and being more careful with my throws vastly increases the chance that the ball will go where I want it to.

Any Pokémon you don't have, is worth more than repeated evolutions. So trying to complete your pokédex is always a good idea for gathering experience.

After that, you should primarily be concerned with the following three evolutions:

Weedle > Kakuna
Caterpie > Metapod
Pidgey > Pidgeotto

These evolutions cost only 12 candy, while all others cost at least 25. Since all evolutions reward you 500 xp, these three give the biggest bang for your buck.

And if you can, use Lucky Egg to double all xp you get from these evolutions!