I landed in BOM a couple of days ago and the runway seemed to be a bit grippier than I remember - has it been resurfaced? Anyway, I can confirm there was a lot of water about, three aircraft went around due w/s.
...it won't help much to select Autobrake max with poor braking action (provided braking conditions are constant over the whole runway)
I think the proviso rarely applies, as from my experience, the worse the braking action, the more variability there is. Water/slush/snow/ice is rarely evenly distributed and you’ve likely got areas of rubber deposits around the TDZ at either end.
Autobrakes generally go for a fixed deceleration rate on anything but MAX, where they function like the ABS on a car with your foot hard on the brake: they extract the maximum stopping performance available at that instant. Consider a runway where the braking action of 50% of the surface (not necessarily in one bit) is worse than reported and 50% is better - using MAX will get more stopping in the good bits and the same in the bad bits, so a net win over an AB setting that technically should give the same result but may surprise you on the day.
Downsides of using MAX AB? Err, maybe a little bit more brake wear? Downsides of going off the end...