A very subtle question, I like it!
Well, there's an implicit long bias anyway, because most markets have gone up over time I will always have more +20 and -20. And the effect you describe is probably strongest in clear risky assets like equities (or risk off assets like vol).
There is some analysis that goes part of the way to answering your question in these two posts:
https://qoppac.blogspot.com/2020/07/do-non-binary-forecasts-work.html
https://qoppac.blogspot.com/2020/09/forecast-linearity-and-forecasting-mean.html
GAT
Hi Rob, have you considered adding spreads into the system? My trend following portfolio has 30% made up of spreads, mostly calendar with a few inter-market ones (KE/ZW, ZN/ZB, GF/LE). The result is decent. Transaction costs double, and risk of breaking correlation is there. The reduced margin requirement really helps. Some spreads trend better than outrights. I would appreciate if you can comment on this. Thank you.