Since this came up, I say that unless you do events where it would be really useful to know your hour power......it isn't really worth messing with.
Do a few benchmark efforts around the time durations of workouts you might want to do, then take off some % to be able to do the sets. Then adjust the workouts as necessary.
20min for many folks way over estimates things. But can be a perfectly fine gauge if you want to know what power to do your 8min sets or 9min under/over sets at, for example.
Just IMHO.
To address the above about doing 330w for 20min and dying in a Zwift race, sounds to me maybe you weigh more than the other competitors doing more w/kg. Just a guess. If so, for now, tailor your events to flatter stuff where your power shines through and the skinny boys on lower power have a harder time.
The thing about some of the figures floating around out there too that rubs me as funny is when the 20min versus hour figure gets wildly lower than 10%. The old 5% is if you do the actual wringer warmup for the 20min test that nobody does right. 10% for many folks seems right. 30% of my 305w 20min best would be about 275 or so. I can do that, now. It's in my Trainingpeaks that I did.
If your hour seems impossible at even 10% reduction....I'd say it's something to look into.
As for the indoors/outdoors thing, I find that power loss is if you're talking longer indoors efforts. Like 20min and longer. Under 20min, or in sets with enough rest, I feel I can cool off enough to go again. It's when performing at top level indoors for loooong periods the heat just doesn't dissipate enough due to no rest intervals. Outdoors, the volume of air is just so great that doesn't come into play.
So, IMO, I'd say no.....my 20min indoors is not lower than my outdoors. It's just short enough the heat accumulation doesn't kill it. The advantage indoors on 20min is you don't have to find a route where you can put out a super steady power for 20min straight. I don't live near big climbs. Shoot, many country roads here have stops or lights after 5mi at most.