I'm new to Emby, but I've been testing the crap outta x265 over the past couple weeks... I even let a 48hr x265 supa slow encode complete to see the result. Don't do that!
As for your original question: Generally it's a bad idea to re-encode your x264 into x265. the quality loss might be fairly small, but there will be some loss. Of course, if you have 4,000kbps+ x264 1080p video files, go for it... Just stay between Constant Quality 24 and 18. You might have to try a few times to find the sweet spot for your encodes. And download the Handbrake Nightly, it has an up to date x265 encoder that beats the last stable release by a mile (losta improvements between v1.5 and v1.8).
StaxRip lets you update x265 encoder on the fly, btw, but it's got a bit of a learning curve. I tested both, Stax would be preferable if you want to tweak the hell outta your x265 encodes. Handbrake allows command line passthrough, which is currently undocumented, but it's otherwise very limited.
You didn't say what kinda quality your original files were. more info gets ya better answers. I could give you a good idea of what Bitrate to shoot for with x265.
Personally, I'm okay with sub 4,000kbps x264 archive video, so I rip BR-2-x265 at Constant Quality 25-22 Medium Main, with 5.1 HE-AAC. I get between 15-23FPS for the encode on my i5 6600k, that's less than 4 hours (as long as I don't use any filters).