I've tried hundreds of config tweaks for it on my fairly low traffic forum and it always results in the end conclusion that it breaks parts of xenforo. Sometimes only after several days nginx uptime, sometimes only for certain users, but the cause is always ngx_pagespeed.
Might be worth another try when it comes out of beta, but for now I wouldn't recommend it with XenForo. Any other website/web app I've tried it with hasn't had any issues that weren't directly attributable to a certain filter.
As for resource usage, once the cache settles down (i.e. it stops resampling images) nginx cpu usage is only slightly higher. RAM-wise, you need to allocate a gigabyte or so of tmpfs space to it.