Very interesting post by Old Smokey, allways a pleasure to learn something.
Actually we do have the same caution in our manuals for the derate case, however we do have at least a Vmcg table for each thrust setting so we can see if we are Vmcg restricted with our V1, sadly there is no information about Vmca (737-700). We dont use derate on our -300s allthough it is available, no clue why.
I dont know if its true, but using derate, assument temperature and improved climb can lead to pretty high V-speeds (V1 past 150 and Vr well past 160) on long runways, is it safe to assume that with such high speeds controllability is actually pretty much a non-issue if we get the dreaded engine failure after V1?