If you're interested to see how Rock Band 2 and Guitar Hero: World Tour's 16 shared songs match up to one another, look no further than YouTube. While there are certainly others (and undoubtedly more to follow suit), user Gurra100 has a playlist of comparison videos (embedded after the break). Everything we've seen so far looks unsurprisingly identical (update: scratch that, we just watched "Go Your Own Way" -- not so identical at all).
Take note how GH:WT handles a Rock Band 2 set -- namely, it's scaled down to a 4-pad note chart. According to some on the Rock Band forums, trying to play the 5-pad variant with a RB2 set plus cymbal pack leads to glitches.
Meanwhile, our Instrument Compatibility Matrix has been updated to version 3.33 and we're still working to fill in all the missing pieces; we've reached out to Activision and Harmonix for help.