Top temp on my FEC100 is 424, though I've never verified that with a probe.
While grill grates will retain heat, I'm not sure where the extra 100-200 degrees come from...they can only get as hot as the internal temp of the smoker.
My 1st pellet smoker was a Trager Lil Tex, which claimed to be both a smoker & a grill. In theory, it was but I wasn't ever happy with the grilling results...just not enough heat for quick sear/grill marks we expect on a steak or chop.
Given your ongoing issues with the FEC, perhaps you should consider a Fast Eddy's PG500 or PG1000, which are designed as true smoker/grills. Unless you're in need of the 100 lb capacity of your FEC, the PG's will give you the flexibility for hi temp searing & lo temp smoking.