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If you pay it off, you will NOT see an increase in your score and it will be hurting your credit report for 7.5 years (or three more years for you). Believe me...I learned the hard way that once you pay them, you are pretty much SOL.
Credit rebuilding is about leverage. Banks and Collection Agencies don't really care about if you're a nice guy, or if you didn't mean run up a debt, if you volunteer at an old folks home...they want their money back. In other words, you have something they want. However, they have something you want (ability to delete thei TL from your credit). So it's time to play Let's Make a Deal!
Try to negotiate a settlement with them. Make them an offer for less and if they accept, do it in exchange for a deletion from your credit report. GET IT IN WRITING and don't send money until you get this agreement in writing. They may threaten to sue, but for this amount I wouldn't worry too much about it. It's in their best interests (and yours) to settle.
Edit: Whoops I didn't read the end of your post. Well...since you've already paid them basically, you have lost all leverage :-(. GW letters work sometimes (I just had one work). But I won't lie....it's a long shot. Call them up and see if they can help you. Good luck!