apple mac refund under warranty?

Sorry mate, the terms of the warranty on most electronics (not just Apple) state that if a machine is defective the manufacturer can either repair it, replace it, or refund it at their sole descretion. The customer doesn't get a choice over which option is picked.

If they are choosing the repair option, there's not much you can do about it unless the repair doesn't fix the issue.

Your Macbook Pro has probably become sluggish because you filled too much of the hard drive up. OS X requires at least 10% of the hard drive space free in order to work at peak efficiency. When it drops too low your computer can crawl practically to a complete stop. Windows is probably having the same problem. Take a good look at what you have on that laptop and start freeing up space to by saving to an external hard drive instead.

You may have different laws where you are, but most companies only have a 14 day no questions return policy. You are going to have great difficulty convincing any company to accept a computer several months old. Your warranty does give you free repair though. So get it looked at. I seriously think that it has more to do with user misunderstanding of how the machine works than any hardware fault. But you should check out all the possibilities before disposing of your Mac.