I did this a couple of years ago. I sold my my MacPro and used a MBP.
I had both and after looking at my situation, I thought it was rather silly to have a MacPro for my usage. I sold it near the price I paid for it, and went over to the MBP.
Storage was the biggest adjustment I had to make, I had a TB of internal storage, which I had to go to external storage on the MBP.
The MBP was slower but for my needs and usage it really wasn't all that noticable. I'm a light occasional PS user, a medium Aperture user and I need to use windows either in bootcamp or vmware so in those situations, I did not see a huge performacne penalty.