Well, the method you used is excellent and just fine for comparing 2 optical trains with 1 film, or one train with 2 or more slide films with identical contrasts, but in this case you are comparing a slide film with a contrast of about 1.8 and a negative film with a contrast of about 0.6, so how do you normalize the results?
And, even though you get a difference, how do you explain the data presented by Kodak and others which says that the Ektar is sharper than the Ektachrome?
I have personally done lab tests (minus optics) which give negative films the lead when the contrast differences are ironed out. Also, and in any case, given identical optics the degree of degradation should be comparable between the films. Thus the Kodak data should stand on its own merit.