This discussion seems to be inconsistent with a long history of posts on this forum by licensed electricians. In these posts, the electricians have pointed out that the purpose of a breaker is to protect the wiring, NOT the device, from overcurrent situations. Therefore, the maximum breaker size is driven by the wiring, NOT by the nameplate amperage on the device.
Previous posts have stated explicitly that the manufacturer of the device is required to install protection within the device to handle electrical issues such as overcurrent situations, faults, low voltage etc. For example, my GE wall oven uses 16 gage wire for a 30A circuit within the oven. I found this curious, as 16 gage wire would never be used in your house, and if it were it would be rated for perhaps 10 amps. On this forum, several electricians explained that within the stove, the manufacturer is free to use any type of wire they choose, so long as the device is rated appropriately by whomever rates the product, which is not NEC. So in my case, I have 10 gage wire connected to 16 gage wire, which is perfectly OK apparently, so long as the appropriate connector is used (the connector is listed in the installation manual, happens to be a split ring type of copper connector).
So my take is that based on past posts, you need a circuit with a minimum rated ampacity of 30A, and a breaker with a minimum rating of 30A, unless your manual specifically states that the breaker shall be exactly 30A. In that case, the manufacturer would be relying on the breaker as an overcurrent device to protect their oven, which I suppose they can do, but would be inconsistent with previous posts regarding the obligation of the manufacturer to protect their device independently of the breaker.
If the breaker is required to be exactly 30A, you could simply replace the 40A breaker with a 30A, as previously noted.