Odoo 8 use report module by default and it's not using webkit_path. webkit_path is used in report_webkit module. Historically, odoo 8 move to support generating report with wkhtmltopdf by default and implement it in report module. The report module itself use PATH environment variable to determine the wkhtmltopdf binary.

I'm not a fan that use PATH environment variable. So, I create this patch [1] for odoo 8 that add webkit_path option to odoo's configuration. To define path to wkhtmltopdf executable define webkit_path in configuration file .conf, like so.

webkit_path = C:\progra~1\wkhtmltopdf\bin\wkhtmltopdf.exe

[1] https://gist.github.com/bbn-bernard/b87b24d23006c29978f5

Seriously: move to Linux.  On the side node, check from your command line whether wkhtmltopdf --help returns anything.  If not, you need to include the path to the executable in your PATH environment variable.