Thirty-some years ago a friend did some bullet testing with a 375/8mmMag using a few powders, but mainly H4831. the loads were right at the top of his rifles maximum loads. during the course of the testing, one day (in Northern Idaho) was 80+F and sunny. Most of the ammo was in the shade but a few boxes were exposed to direct sunlight. when it came time to shoot the ones that had been in the sun there were immediate signs of higher than good pressure. Primarily, brass swaged into the ejector slot which the bolt shaved, leaving a very bright spot on the casehead. Those boxes were heldover for a cooler/shady day and performed back at the usual no extreme signs. the conclusion was that the H4831 was susceptible to temperature related variations. Since those days in the early 80's Hodgdon has introduced powders, including H4831 that are listed as temperature "Extreme" and is advertised as being affected very little by variations in temperature. I know of no independent testing of side-by-side loads using the old and new powders to show a comparison, or independent testing to show how cartridges perform when allowed to sit in the sun for a couple hours.
But as noted previously, if the 450/400 is loaded to original performance an increase in pressures due to temperature should still be well within the safe limits for the cartridge/rifle.