I agree with Luri & Brigita. They are different and certainly is broader than a company walls. Since the scope is relatively large including product design, supplier selection and material sourcing, manufacturing processes, product packaging, delivery of the product to the consumers, and end-of-life management of the product after its use.
See the following paper, hope it helps.
Diabat, A., & Govindan, K. (2011). An analysis of the drivers affecting the implementation of green supply chain management. Resources, Conservation and Recycling, 55(6), 659-667.
Agree with Luri. Green Supply Chain is something what a firm follows and its outcomes or the metrics is the performance of the firm. So they are different.
In addition to Iuri's comments, Green Performance of the firm/plant can encompass all the environmental performance activities that a firm undertakes such as switching off the light, saving water, recycling office materials etc., which may or may not be part the Green supply chain activities.
I do believe that they are different as other researchers have mentioned, but for a better understanding you can download two different papers concerning green practices and green performance measurement.
You can refer to a paper authored by me and of my colleagues.