I found this great little plugin that lets you change the default WP JPEG compression level: https://wordpress.org/plugins/simplejpegquality/ which is a great thing since 60-80 is really ideal, but WP defaults to 90.
However, it doesn't have an option to set it up and set a single setting for the entire network on Multisite. You currently have to set it on each individual site.
I asked the Dev if he would update it to do just that: http://wordpress.org/support/topic/feature-request-network-wide-setting-for-multisite as you can see, he said he'd give it a shot, if I could provide more info on how to do that.
Since WPMUDEV is kinda THE source for Multisite plugins and expertise, I thought I'd ask if anyone here knows a good article or other documentation that might help him do this? It really just needs to add a setting under Settings>Network Settings that applies to all sites on the network, much like Limit Post Revisions plugin does: http://wordpress.org/plugins/limit-post-revisions-network-option/
I'd really like to see it done, and most Multisite admins should really consider this, if/when he updates it. It'll help sites load faster, with smaller JPEG images. Thanks in advance for any advice, referrals, or other help!