Sig obliquely raises a good point ie. iMacs do not travel well. The issue you have to decide is performance versus portability. You would gain a lot more performance with an iMac for the work you are doing than with a newer MBP and it would be less expensive!
You might consider installing a SSD to speed up your current MBP. This might be one of the few instance that would make them cost effective.
Look at this link. It might help you gage what kind of performance increases you can expect. The OWC website also has bench test results