Does XtremeTuner Plus support GTX 900 series? (GTX 980 HOF owned)
No, the software doesn't have support for GTX 900 series. I've checked the official website, and it appears that you can us it for 700 and 600 series. For this type of applications, I wouldn't recommend using them unless they're full supported with you graphic card or you might break its functionality by damaging the BIOS and other virtual aspects.
You can stay in touch with updates by visiting the following page: http://www.galaxytech.com/__EN_GB__/Driver2/Xtreme?areas=__EN_GB