The short answer is that the new 95 inch boom can be used in applications which require lower measurement uncertainty than the 60 inch boom.
As for the long answer...
I believe you might be looking at Figure G.5 in IEC 61400-12-1 which is based on 2 dimensional Navier-Stokes computations.
If so, the figures you indicate are about right (4 tower diameters equates to about a 2.3% centerline disturbance). Another consideration is that the IEC model measures the offset distance as tower centerline to sensor centerline, which really places the 60 inch boom at about 6.5 diameters (for a 10 inch tower) equating to a centerline disturbance of about 1%.
However, here is another way to look at it...what level of disturbance is acceptable for your wind resource assessment campaign?
Many in the industry would say "let?s make it 0.00%!"...
Basically, the further the sensor is away from the tower, the lower the disturbance. In reality, the law of diminishing returns comes into play and no matter how long a boom is (realistically) the disturbance will not (in theory) be 0.00%. At about 10 diameters, the disturbance curve flattens out and approaches horizontal. This is really the sweet spot of boom design as the centerline disturbance is less than 0.5% (some might say it is about 0.3% but I prefer to be a bit conservative). I have also heard that some in the industry believe disturbances are actually greater than what the Navier-Stokes model predicts. If anyone has seen research papers in this area, please add links to this thread!
Of course, other design factors are involved such as robustness, cost, ease of installation and transport (the new booms are cost effective, easy to install and are also UPS shippable).
In summary, NRG will continue to offer both the 60 inch boom and the 95 inch boom. The new 95 inch boom can be used in applications which require lower measurement uncertainty than the 60 inch boom.