What machine? What HP, and what sized deck?
It depends on how the mower was set up by the manu regarding pulleys, etc. My Deere Z's came with .250" X 3"W high lift blades on the bigger one, and the other came with .312"x 2.5"W medium lift blades. Both 60" decks. Both run/ran the thicker blades just fine. If your machine is/was originally set up/designed to run .205" high lift blades, it may lose some power with the .250" high lift blades. More so if yours came with medium lift blades and you try to run thicker high lift blades.