Can't say for R9 300 series, but from my experience using RX470, the GPU can help during editing, but for Mainconcept GPU assist rendering, not working.
GPU-rendering only supported on older cards: GTX 5xx or AMD HD 6xxx.
I'm using an R9 380 (8gb) card. It is recognized within Vegas Pro 13, but provides no render speed advantage. I think it does help with the preview playback speeds. With this card, these are some comparison render times with a 1 minute 1080p project.....
MainConcept render takes 1:30
Sony AVC render takes 0:58
I get faster results and better quality results using the Vegas2Handbrake process, which takes 1:04, including the 20 seconds overhead required by the scripting and FrameServer process and scanning the source into Handbrake.
If I send a 2 minute 1080p project to Handbrake the render time is only 1:36 including the 20 second overhead.