I feel as if my boat needs a -2 or -3" lower unit. I am currently running a std length imco lower, with a 5 blade P5X. What is the best way to tell if the shortie is the way to go?

Really depends on the bow lifting characteristics of the boat. on my boat, a single step, pad bottom I am at 3.25" below the hull, and would like to try another inch up. Some other boats need the bow lift that a deeper prop provides.

What makes you think you need to go shorter? how deep is your prop shaft currently?

3-5" below the bottom is the norm.