The short answer, I believe, is for the level changes you're talking about (which if I did the math correct is changing from 1.5 qts/lb to 1.2 qts/lb) is that you wouldn't notice much difference and you shouldn't worry about it.
Kai Troester did some nice experiments on mash thicknesses http://braukaiser.com/download/Effects_of_mash_parameters_on_attenuation_and_efficiency.pdf
. His results, and the results of others he cites, suggest that mash thickness has little effect on attenuation. He did see a temperature effect of mash thickness on overall efficiency. He was comparing mashes done at 1.2 qts/lb and 2.4 qts/lb and found that the thin mash had better efficiency at higher temperatures. I suspect that for the amounts you want to change and the temperatures you'll most likely be at, batch-to-batch variations in your efficiencies will depend more on your measurement accuracy than changes in your mash thickness.