Gonzo- Very true. But I don't think that people should think that there either (1) no gains in performance, even in the short run, are possible, or that (2)if they are not feeling great on day 1 they do not have the possibility of improving on days 2 or 3. Certainly no one can live at sea level and maintain the same level of performance after a rapid ascent to 9800ft, but even acute acclimization over 48 hours will result in some improvement in both performance (tissue oxygen utilization improves in days, not weeks, even though hematocrit takes weeks to increase) and in parameters like sleeping better, relief of headache, etc. Furthermore, lack of such improvement should be taken as a warning sign that the person is either not acclimating well (and should take it even easier, drink more fluids, etc), or is developing worsening AMS, in which case they need to consider descent before they get really sick.
Studies have shown that those who have attained excellent cardiovascular fitness at sea level DO perform better at altitude than those are not as fit, so all of that conditioning, even in Brookline, is helpful (but not as good as were it to be done at altitude).