My GI program did not require taking Medicine Boards a certain year... As a GI fellow I will say, Internal Medicine knowledge slowly falls from your memory by the day. You can try to fight it, but it happens to everyone. Get the ABIM out of the way. You will never be as strong in internal medicine as you are when you finish your residency; maybe going into other subspecialties, but not GI.
Another point, you need to have passed IM boards prior to sitting for the ABIM GI boards. God forbid you do not pass the ABIM after waiting until your 3rd year; you will not be able to sit for GI boards for another 2 years (have to wait a year to re-take ABIM). Just get them done. Your first few months in GI are basically just learning your way around the hospital, how to hold a scope, and call is light.
Just my humble opinion though.