That's normal to have calls through to iPad (it was on iOS 8) as it routes through the iPhone, this new iOS 9 option lets you make actual wifi calls from the iPad (or Mac) without the need to route through the phone - it can be off or left at home and you can still make wifi call from another iCloud device.I've got wifi calls enabled and add wifi calling. Just tested and I'm getting calls through to my iPad. Too early to tell about battery life but not seeing any ridiculous heat.
That's what this new option/button is supposed to do, but it only appears to work on t-mobile in the USA so far. I was wondering if it worked on EE, however it appears not.