Worthwhile to reformat and re-install? I had a 2 1/2 year old Macbook Pro.  It was a 2.33 Intel Core 2 Duo with 4 GB of ram, 320 GB hard drive, running Lion.  

An employee of my company left and his Macbook Pro was a 2.66 Intel Core i7 with 8 GB of Ram, 500GB hard drive.  

At the suggestion of a friend, he said to swap my hard drive out and take the new computer, so I did it...  

I find the new computer is running slower than my old computer for pretty much all the applications (and including booting up).  

Would it be worthwhile to reformat the entire hard drive and do a clean install?  Side related note: Would adding an SSD help this as well?

Thanks
Your best bet is to restore the original hard drive, it is most likely much faster than the 2.5 year old one, and then use Migration Assistant to Transfer your files to the new computer.