I use the filter in Gimp-2.8.1

And i have had no issues at all.
You need to be sure you create a text layer (do not rasterize this layer., it must stay a text layer), and then a separate path or paths layers.(your shape)
Be sure to start the shape of any of your custom paths at the TOP-LEFT corner and use ONLY 4 points.
Top-Left
Top-Right
Bottom-Right
Bottom-Left
back to Top-Left and use ctrl + left mouse click to connect the path shape.
Select the text layer and run the script.
It takes a good while to run even with 4 letters. A simple GIMP text on my 1gig ram machine took 10 minutes.
You will see each shape or text letter become a bunch of nodes as the letters are created.

It is an excellent script but can be quite slow with many letters or a big envelope.

-Rod