This reduction from 40 degrees to a maximum of 30 degrees was applied to a range of Cessna singles. There were a number of landing and particularly take-off incidents that were attributed to the enormous amount of drag produced from a run on to 40 degrees which had been inadvertently allowed. I never found 40 degrees itself an issue but the electric flap switch operation was. With some models you had to actively neutralize the switch position, whether raising or lowering the flap or it would run on. Cessna did change this to a system where it became necessary to maintain pressure on a spring loaded switch or it would reset to the neutral position. The flap indication was poor which could have a lot of lag. They then changed this again to a gated system allowing the pilot to select the required flap position.
The excess thrust over drag was marginal with 40 degrees of flap and required skilled and accurate flying in order to climb.