Looking for some advice on cooking the puff pastry in Beef Wellington
My concern is the puff.  Generally, to get great results,puff depends on the heat to separate and fluff the layers.  I would suggest the following:
1.Get a piece of the puff pastry you will be using, and cook it at 275 to see if it puffs, or looks sort of flat like a Napoleon.  At the same time, wrap a bit of your duxelles in the same pastry, and see if the moisture changes your result.
2. By all means, use an accurate thermometer in place to get your meat to the doneness you want.  (Thermoworks.com has some good, accurate, inexpensive ones)
3. If your meat is not getting done by the time the puff is ready, consider partial pre sous vide of the meat.
3.  As the first answer, I don't see why this change will improve your results.  Lower temps generally produce more tender meat, but you are starting with filet, which is already very tender.
4.  The last concern is that at the lower temp, you may end up with a soggy bottom.  If this is so, then reduce the amount of residual moisture in the duxelles.