My understanding of the RYA advised PY adjustments due to keel configurations is that if you have the POTENTIAL to lift your keel then you MUST race on 976.
Of course your club may be at liberty to allow the variation you mention.
If so, my feeling is that with the keel down, off the wind, the boat IS probably slowed by at least the ca.1% advantage you would be getting. So it would entirely depend on the length (in terms of time)of the downwind leg as a proportion of the whole race to make the change worthwhile.
However, in my experience of racing, it is not always advisable to raise the keel significantly in a blow downwind ... in such circumstances you would get a huge advantage at 986.
I am surprised that you are allowed the option.
Chris Turner