Derivative works, copyright questions!

[mctrav]

Hey there,
Have done a bit of searching on the topic, am looking for opinions / experiences.

I want to record some popular songs on piano, say a Lady Gaga song, 16 bars of 4, "verse/chorus"
To make a CD to put on itunes for dancers to dance along with.

After some searching, I think they'll be "derivative works" and not covers.

Anyone else ever recorded any instrumental covers, contacted the original artist for permission?

Or, obviously this is an extremely subjective question, how much do you have to change the melodies etc?

Anyone had any experience in this area?