OSX, Metal, and External GPU

edit - update in comments

So, I'm running a 2017 MBP 13" with an RX580 connected through an external GPU with a 1440p monitor. Specifically, it's the Dev Kit that released a few months ago. I can do WoW at 1440p on medium setting at 60+ fps in most situations, but everything else I play is pretty casual and non-intensive.

I tried LotRO and I can't really break 40fps, and even that's on an unplayable 720p resolution at minimum settings. The game is "recommending" ultra settings (perhaps based on the GPU alone?), but that gives me 20fps at 1440p, and about 25fps at 1080p.

Anyway, does this game just run like trash on a Mac? Is it a problem with External GPU support? Does it even use the Metal API? I honestly figured I could do at least as well as the latest expansion for WoW, including all their updated animations and graphics, but LotRO is basically unplayable.

Anyone have an idea?

Also, has there been any communication regarding a 64 bit client? With Apple phasing out support for 32-bit applications over the next year, it seems like a pretty big issue.