Yep, the hacks should be fine. I have:
- Jailbreak
- Custom Screensavers
- Screensaver Timeouts
- Launchpad
- USBNetwork
and all of them are working well after the 3.1 update. I don't have the fonts hack, but I'm betting that it works fine.
I don't feel that anything is snappier, and I'm quite disappointed that they haven't added support for links that open in new windows for the webkit browser. However, I absolutely love the new articles/section list thing. It's so much nicer looking and easier to use. I can't comment on the page numbers because I haven't noticed them on any of my books. (most of mine are mobi)
Anyways, the bottom line is: If you already have all the hacks installed, go ahead and update. There's nothing bad about the update, and it adds a few nice things (though it leaves a lot to be desired).