Not too sure how that works on RE Shocks, Not the best initially and age wont have helped.
I have a pretty good idea of how suspension works and RE is fairly archaic, so my upgrade was my best and easiest informed option.
£65 posted and problem solved
From my GSXR Gixer Days I dont think that would help, if the shocks to short or soft its NFG, RE Shocks are regrettably not exactly state of the art LOL, My £65 replacement worked wonders
Lets talk about the elephant in the room shall we so we don't confuse the OP as you seem to contradict yourself in the above 2 quotes.
Preload is exactly the same on either a gsxr or a bullet. the difference being a gsxr is a mono shock and linkage and the Enfield maintains the traditional twin shock lay out.
Preload adjustment has been the standard way of adjusting springs for extra weight / luggage (without going into compression and rebound damping) since the first adjustable shocks were available and in some cases such as a velocette venom and other classic bikes you adjust the mounting position of the shock to do the same thing.
As others have said you increase preload (which 'pre loads' the spring) to allow for extra weight which in turn raises the rear ride height and reduces the static sag you get when both the bike goes from full extension of it's shocks to bearing its own weight and further reduces the sag when rider is included in that mix.
Even knackered springs will respond to preload adjustment when the bike is on the stand, so only if the spring has had it would you see no improvement to how the bike rides with pillion and luggage so the preload test would rule that out.
longer shocks sometimes come with a softer spring so may not be of any use to the OP in this case and personally what's to say the 2nd had pair of shocks would be any better in condition as you wont know their true history.
If preload doesn't work and a new set of suspension doesn't cure the issue, and the bike is put together correctly the only way forward would be to fit a firmer spring or have a bespoke set of springs and shocks made up to the OP's specification.
That's not to Poo Poo what worked for you in anyway mind, just don't want the OP getting confused.