hmm good point, but in terms of sidious vs vader in a saber duel match i would say vader has an advantage using the shien form because of his brute strength, in terms of physical strength vader can crush sidious because of a quote in rodv "he has enough strength in his mechanical hands to crush his lightsaber hilt with ease" something like that
^.~ 1 thing, OT vader is a far greater duelist than ROTS presuit vader, hes slightly slower but has greater strength and he is limited to the shien form, because the OT fights were slow people assume vader is as slow as hell but in a flashback in crimson empire, he seemed agile and fast enough to chop his opponents hands off with a vibro sword.
o ya you also mentioned sidious hasnt dueled any1 after yoda for 20 years so yea its obvious his skills get sloppish and not as good as before, same thing, vader plays with shien form for 19 years and knows its ups and downs,
for saber duel id hand it to TPM sidious who is above DE sid and ROTS sid in terms of saber combat because he stills practises with a saber, after rots its 24 years of not playing with the lightsaber which is the time of DE