Gotcha. IME…Going from gatorskins with slime tubes to GP5k with butyl or latex you will notice a difference in speed for the same power with no special equipment other than your head unit. Total WAG from my numbers but, 40W at my threshold on my TT bike is around 1.5mph. (320W=27.5mph and 280w=26+ a little mph). So I say go out and run the slow tire with slow tube then run the fast tire and fast tube and find out.
I could not notice a difference in speed on my head unit at about 10w or less. Just noise.
While I didn’t Chung method it I spend enough time on smooth flat unobstructed roads in no wind similar temps to know. I think between running a GP5K with butyl and latex would be in this same ballpark. Not saying there isn’t a difference just that you won’t notice it on the head unit. Some of these little gains are only measured over long distances in seconds or fraction of seconds/km.
Latex feels maybe a little different (cornering) and they are actually more durable but, just a little more care to install and take care of. Lightweight butyl I’ve never felt necessary or comfortable with the possibility of a blowout. So I generally run butyl for training and racing. If on the TT bike for a race I’ll usually install latex a few day before and just run them til they die and put butyl back in until the next race.