After trying to guess the transfer info by using SSIM, I figured out that the most accurate result was "240m" (SSIM=0.999999992133779).
Thanks to that, I could make some testing... and here are the results:
Box filter, smooth=0.75194, SSIM=0.931384846830042, srgb
Box filter, smooth=0.317681, SSIM=0.970661757426569, linear
In an attempt to make a conclusion, the linear and srgb results are somewhat disappointing, as the srgb ends up being neutral, because the darker and lighter tone are more preserved than the neutral ones, the latter ones are smoothed. Meanwhile, the linear one is slightly overbright, the darker tones are worse preserved in favor to preserve the lighter ones.
EDIT: Yeah, I pointed out what
Robidoux did pointed out years ago,(if you want to squeeze out your source, may as well use linear+cie primaries), although, by the time you figure out a source's transfer and primaries color data and converted them to linear rgb+ciexyz/ciergb, would be simpler and more versatile to just use area resize+SSIM downscaler.
Atlhough, It may be benefitial for some scripts and sources, I wouldn't recommend it unless you have enough resources to squeeze and enough patience and time to figure out all your source's color info.