Is there a performance impact for capturing additional data in nginx logs?
The performance impact would in most cases be minimal. A quick scan of the source shows that the values of those variables are calculated internally regardless of whether they used in the logging so the only overhead is the additional time to append those values onto the log entry and write to output - usually a disk based file.
The additional time is going to be minimal compared to the overall processing overhead. This is also my own experience of logging response times just as you are suggesting.