Mastitis is an infection. It does seem like pumping and dumping would be better than just letting the milk sit in the ducts, since milk is a great medium for growing bacteria. But you wouldn't have to pump as much as you would to feed a child. I hope you get a better and more comprehensive answer than this from some other poster! Also, try posting over in the breastfeeding forum on this site. Good luck!
Mastitis is really awful, I feel for you. It is very important to completely empty the breast and that is why pumping and dumping will work. It also works to clear out any clogs too and you can set the breast pump to a comfortable level. Mastitis usually starts as an obstruction to the milk flow and then when the milk has not been removed it goes onto being an infection or the formation of an abscess. It also can be germs that from the baby's mouth,nose or throat into a clogged milk duct. The key is to empty the breasts right now as much as possible, get as much bed rest as you can, make sure you are eating properly and get others to help you out too. After the infection dies down, then you can go back to slowly reducing your supply and stopping breastfeeding. Now would not be the time to stop cold turkey, it'll make your symptoms worse, but when you are over the infection, reduce your pumping and dumping and your breasts should heal up nicely. The other things too that have helped me is a warm compress or warm shower/bath before you pump, massaging the breasts and ibuprofen. You'll feel a ton better too when the antibiotics kick in- make sure you take them for the full course and if you don't feel better, make certain that the doc gives you another round.