When our EF Roo died, we performed a necropsy and his skin and meat looked like any other chicken...just not much meat (they are a lean breed). All the crosses we have processed (EE, Aust., RIR, WLeg, BR, etc...) have had normal looking skin/meat. We still have EF genes in our flock for pest and heat tolerance, and the only difference (aside from plumage) between our birds and the straight lines is that they are leaner and flightier.
The heat tolerance and disease resistance is what has prompted my interest in them as well. My Delawares do very well here in North Florida, I cull anything that can't hang, but I have been thinking of introducing some EF genes and then breeding back to another Delaware or New Hampshire Roo. Have you been satisfied with the results of introducing EF genes in your flock?