Right, first of all Estate rear springs
will fit saloons. It's the same car/chassis/floorpan etc. Further borne out by the fact a mate of mine ran his Elite with Estate springs for a year or two. It is/was a common mod for local taxi drivers to stick the Estate springs on the saloons, as this got round...
-three generous-sized rear passengers
-generous-sized punters plus their luggage
-local speed bumps
...rather nicely.
I am sure that when blitzed round a test track The Stig will be able to reach the conclusion than a completely unladen saloon on estate springs will feel a fraction more 'floaty' or 'bouncy' at the rear, but once you have some luggage, rear occupants, or... a caravan to tow, I'm pretty sure that Silverstone Trackdays are out anyway

Rewind a few years - before the advent of 'self-levelling rear suspension' - and you'd be advised to fit stiffer springs, or an extra leaf if you intended to tow regularly anyway.

As for standard estate springs
combined with self-levelling shocks that may not work so 'happy'. The SL system will constantly read the back axle as high, and try and compensate. The proper SL springs are softer, and of course lower than estate.
Now as for
MV6 estate springs, I'd presume they're lower than normal estate springs, but still a little higher than saloon MV6.
But really only SL springs want to be used with SL shocks. SL springs
alone, with
normal shocks tends to leave the rear end rather low and 'soggy'
