For mechs that are already mostly maxed mythical, yes, it's a big factor. For everyone else though, just having one more mythical weapon or legs or drone than your opponent has a similar impact.
Furthermore, as Toxic and I have mentioned in other threads, there's a huge random/who goes first factor that often overcomes a 200 HP difference.
A good example from my own games right now is when I face Frogbot, who I've played against like 10x in the last few days. He wins about 2/3 of our games cause he's a bit more fused, but with the right starting positions or RNG rolls, I often win by as much as 700 HP.
Before I draw a bunch of fire: YES mythical plates are too strong, and YES having 2 vs zero is rarely surmountable. But they're not the be-all-end-all people make them out to be. I'm starting to beat 2000 HP mechs and I'm 1700 and not that well fused.