How to Improve Site Performance

I'm on mu with buddypress daily and my page speed is terrible. I'm not blaming the theme...I'm hoping you guys will have some more ideas for me.

My YSlow score is was a terrible 49, but I got it up to a slightly less terrible 54.

I was able to get my improvement with Use Google Libraries (which took about 3 scripts) and using WP-Minify, but only for Javascript - the CSS option kills the look of the page.

My worst scores were for these things:

Grade F on Make fewer HTTP requests

This page has 21 external Javascript scripts. Try combining them into one.
This page has 24 external stylesheets. Try combining them into one.
This page has 10 external background images. Try combining them with CSS sprites.

Grade F on Use a Content Delivery Network (CDN)

There are 103 static components that are not on CDN.

Grade F on Add Expires headers

There are 48 static components without a far-future expiration date.

Grade F on Compress components with gzip

There are 24 plain text components that should be sent compressed

Grade F on Minify CSS

Grade F on Configure entity tags (ETags)

There are 38 components with misconfigured ETags

Google Pagespeed came up with these weaknesses:

Combine External CSS
Combine External Javascript
Minimize DNS Lookups (this is the side effect of a hack that pulls posts from sub blogs up to my front page)
Minimize Size Request
Specify Image Dimensions
Parellelize Downloads across hostnames

So far...

I tried WP Total Cache, but it causes but problems with the activity stream and I had to delete it. I may try and bring it back, just checking the "database" option, but I'm guessing there are better options.

I tried to use WP-Optimize to clean up my database, but couldn't even get the admin screen to load without crashing my browser, so I deleted that plugin. I have about 5000 post revisions.

I tried putting javascript in the footer, but it killed my slideshow.

I put in the file-based caching tool recommended in the buddypress documentation, but it then prevented me from being able to add any new plugins. It caused the error message "The plugin does not have a valid header." I had to delete it. For what it's worth this improved my YSlow score by 1 point.
http://codex.buddypress.org/getting-started/improving-performance/

I looked into xcache, referenced on that same page, but installing it seems a bit overwhelming. I can learn it and figure it out...but I wanted to check here to see if it's worth it.

I googled around a bit and it seems like page speed is a common problem with more complex buddypress sites. And, it seems like there isn't any consensus on the best tools to use. Many of the most popular wordpress performance plugins, like DB Manager, are outdated and no longer being maintained.

For what it's worth. I've got a community of about 400 members and 200 blogs. I run lots of plugins, probably more than most. And, we have lots of images. I'm on MU 2.9.2 and BP 1.2.3.

This is becoming a pretty big problem for me and any expertise that you guys can share would be much appreciated!