what would be the effect of operating the existing two runways as close parallels similar to Seattle
I'm no expert, but I suspect
if allowed to do that the movement rate would actually reduce because the space between the runways is not adequate to vacate one withouit infringing the other (never minding obstacle clearance slopes and other awkward factors).
Gatwick already has more passengers than Seattle in an airport half the size. I think they're doing alright with what they have.