Strava calcs for speed is pretty accurate and I also have a Garmin Speed sensor ....when it comes to power, I agree and so do Strava .... Wind and gradient play a huge part and unless you have a power meter, you (and Strava) are merely guessing

thats why I like to compare my rides with others on the same day (ave speed and time on the segments or route) .... this shows me how well I done compared to the others with the same wind conditions (but saying that, wind speed and wind direction changes constanly, but that is the most accurate info at present

and .... I very much doubt that anyone has a KOM ridden in strong headwind on a segment that many use frequently

I have a Microsoft database where all my rides are recorded (not the daily commute rides) .... I record distance, wind speed, heart rate, wind direction, ave speed, time, elevation, climbing speed (I use StravaStix) etc etc

If I want a fast ride for the day, I check the wind direction and speed, then filter my database to show all rides with the same wind direction then choose a route where I rode a fast time

I will be getting a Powermeter very soon, just not sure which one (I'd like one that records power on both cranks