I also have been testing the breeding and have come up with my own evaluation of it. I only bin 2yo's that are clearly crap but it's not very often I bin 2yo's tbh, often certain stats progress when they get older. (You should find your 1m+ 2yo's that are born with little stamina stats and are racing 5f-6f in their 2yo calender will get an increase in stamina stat, for me it's either in August of the same year or the following January i.e when the foal is now 3yo)
I think that Potential, Consistency and Finish Application are important. I also cross breed for stats, for example the first one I looked at in my game,
This is the Dam and Sire
I purposely used two G's with very high or full potential to maximise the chance (not sure if its 100%) of a full potential offspring.
It seems that filly produced has taken the battling of one of the G's but not the enthusiasm I wanted, Also it has taken the
Ultimately, I feel that you can use a horse to try and cross breed certain positive characteristics into another but you also risk transferring negatives too, I therefore have been trying to cross only the most positive with each other, without direct inbreeding (unless I want an anomaly out of the bloodline).
So I look for horses to pair with each other that have the quality I want to add and almost equally as good stats, so that I don't dilute another quality