Is a 450°F max temperature necessary?

Only for very coarse or resistant hair. Mainstream reviewers and stylist commenters generally recommend the lowest temperature that gives one-pass results — typically 300–375°F for fine hair, 375–410°F for medium, and 410–450°F only for thick or coarse hair. Most modern irons in this roundup hit 450°F; using all of it isn't the goal.