You can try it out. Just be sure to periodically touch the extension cord to make sure it isn't getting warm. If it starts to heat up, then you should immediately unplug it to prevent a fire. Manufacturers are going to assume a worst case scenario and typically they over state warnings to be on the safe side. I don't blame them as some people would not know any better and might try to use a really cheap, thin gauge extension cord with cracked insulation around the cable. If you want to be extremely safe, then do your test run outside on the patio with the extension cord plugged into an outdoor plug. But still check it often to make sure the cord isn't heating up.

As far as the searing of meat goes, that is a personal preference. Many people, myself included, like to quickly sear meat on a VERY hot surface. Why? Because it does caramelize the meat surface and imparts flavor. But it also seals the meat. So any meat that tends to dry out while cooking can be kept juicier by searing the meat as that will seal in the juices and impart a lot more flavor. However, if you are cooking the meat in a pot with plenty of liquids, then this is not as important as far as the dryness goes. Again, searing the meat at high heat for a few minutes is just a personal preference, not a prerequisite.