295w target power, 1.16 kg/m3, .0035 crr, .218 CdA, 10mi TT, pan flat, .98 drivetrain loss:
100%, minus 5%, minus 20%: 45.45 kph vs 44.60 kph vs 41.90 kph
200w target power, 1.16 kg/m3, .0035 crr, .218 CdA, 10mi TT, pan flat, .98 drivetrain loss:
100%, minus 5%, minus 20%: 39.41 kph vs 37.89 kph vs 36.25 kph
The 295w power target has a delta of .85kph and 3.55kph.
The 200w power target has a delta of .52kph and 3.16kph.
The relationship isn't a nice linear 5% due to the non linear nature of aero drag at different speeds. For a 40k, the 5% you'd probably be off on accuracy by a whole kilometer over the course of the event (relating the error to a visual cue like distance).
Additionally, in training, if you're doing intervals right on the limit.......being off a hair can mean you don't meet your workout targets. Either underperforming and not adapting or trying to overperform and not adapting because you tried to hit targets you thought you should. The wisdom there is adjusting the target mid workout by knowing your body and how the workout should be going. But most folks do ERG workouts refusing to change the target. Probably the same folks who have the fever dream of thinking their ftp is actually 95% of a 20min test or a ramp that they didn't even perform correctly.
Also, with the linear assumption of 5%, 5% of a larger training target is larger than a small one. So 5% error on a 200w warmup is less consequential than 5% error on a 300w 8min effort where the larger error is more watts. And therefore more consequential.
So yeah, IMO a routinely wrong 5% can toss you in aero or training. Especially knowing most folks don't have the wisdom to hit the little minus button on their training program or control unit in ERG mode when the workout isn't going how it should.