I think you will find that saxophone shops are all too happy to let you try their different pieces. I was in a similar situation to you, Clare. I had had an alto sax for a long time before i took it up in earnest again. After about a year I found the playability and tone was somewhat lacking with the provided mouthpeice and decided to invest in a new one. Those very nice people at saxophones.co.uk set me up with 4 different mouthpieces, and I came away with a lovely ebonite Selmer S80 with a slightly increased opening - E opening (C* to E should be good). The difference was amazing! While you may find a different mpc more suitable for you, I love the Selmer and it makes a very good first replacement for a factory standard on the alto, with a lovely warm tone and considerably better ability to achieve high and low notes.
If you have the chance though, deffinitely do as the above writers have suggested, try as many as you can, preferably on your own horn so you can hear and feel the difference from your original mpc (I didn't have this luxury).
Very good luck to you!
Alan