Short answer - yes
Longer answer - if it's the really old 15' (ie. like the non 64bit core duo model I still have) then it'll be an awful lot faster, otherwise it'll just a fair bit quicker. Obviously depending on what software you're using your mileage may vary but it'll be quicker certainly.
I'm currently considering what to replace my 15" macbook pro with and I have my concerns about dropping down to a 13" screen personally, so I guess if you haven't already, make sure it's for you. Maybe you aren't doing photo work on it but even for browsing on the sofa I much prefer the 15" screen on my macbook pro to the 13" dell I get from work...