The final result likely wouldn't be that much different than WN8, the issue is that right now there isn't a lot of statistical justification for what WN8 does. It uses the same forumla for every tank at every tier, bases performance as a ratio of a top 10% player and has obvious errors as a result.
In short, you are linearizing one too many times and in the wrong place - the 1565-~54% range is not a fair approximation for the 49% range or the 60% range.
As a first step I would like to see the regression which developed the WN8 equation repeated for every tank at tier 10 to see what d