I agree with you there on pretty much every point. However, if a Fanatec wheel is properly adjusted it shouldn't really have enough slippage to matter.
However, now that I look at the graph again, it would appear that the T500 is still accelerating when it hits the reversal call, (the slope of the graph is still very high, where the CSW looks to have mostly leveled out) so it probably never gets near its top speed. On top of that, it looks, upon closer examination, as though the T500 does achieve a slightly higher peak speed. It almost looks to me as though the T500 could benefit from a slightly higher gear ratio, to allow it to accelerate a little faster.