This is a question for the experienced AdSense users... do you find that referral ads do well? I have just put up an ad to get users to register for a free e-book on learning guitar. it's relevant to my user base as the site focuses on an acoustic artist (newton faulkner < my site) i have just removed the 250x250 general ad block in exchange for the referral ad. i am really expecting this ad to perform less, but it's worth a try!
It really depends who you talk to. Some people make a lot from referral ads. Since the ad is relevant to your user base you could possibly see some conversions. Try it out for a couple days and see how it goes, you never know!