Hi. I can answer your points one by one.
1. Rooting is a great way to get 2.1. Of the two options available to you (leak or root), many would point to rooting as the better way to go. Currently, the people who upgrade to the 2.1 leak (without root) are kind of stuck where they are until the official OTA update from Verizon. There is (at the time of this posting) no way to downgrade to 1.5, or to gain root if you upgrade using one of the leaks. The developers are working hard to fix this, but it hasn't been accomplished yet.
2. You could go either way on this. I have heard from people who have upgraded using the leak who have complained that dialer lag is still an issue, and I have heard from some saying that it is much faster. The same split occurs with people who upgraded via root. There are some tweaks you can do either way (disabling the 'touch tone' sound and keeping your call log trimmed, for example) to keep it at a minimum, but it seems like some people have problems while others don't.
3. There is (again, at the time of this posting) no way to use apps2sd for the Eris, because HTC hasn't released the latest kernel to us. Many developers seem to believe this is not as critical a feature for us, since we have much more internal memory than, say, a G1 did. Also, without a Class 6 (fast) SD card, many apps would run much slower due to the slower data transfer rate between the phone and the SD memory.
However, with root access, you can uninstall unwanted stock apps (such as Amazon MP3 and Footprints) to free up more internal memory.
Lastly, the only way to update to 2.1 without having to reinstall all of your apps is to wait for the official OTA update. Your purchased apps (and your contacts) are both connected to your Google login, so you won't lose everything completely, but you will have to spend time reinstalling all of your apps and settings.
I know this post is long, but I hope this helps answer your questions.