real world examples on my 120Ys:

the lowered 2dr sedan is approx 48.5mm lower than standard with an additional reversed leaf added to the top of the pack.

ROLL CENTRE


120Y leaf spring Roll Centre height - low vs stock
stock blue coupe on left - RC is approx 38mm above the axle
lowered yellow 2dr sedan on right - RC is approx at the centre of the axle. String line is distorted from straight by the axle housing.
lowered car's RC is approx 66mm below standard, approx 285mm from the gound with the 185/60 R14 tyre.

INSTANT CENTRE


120Y leaf spring Instant Centre location - low vs stock
'L' length between front and rear spring mounts on chassis is 1140mm.
3/8th L = 427.5mm
= approx 97mm from front eye longditudinally.
Approximate IC locations are at the end of the tape measure.
Stock blue coupe on left IC is approx 80mm higher than the lowered yellow 2dr sedan on right due to a combination of lower RC and decambered front half of leaf.
Effect of lowering is less bind/twist on the spring due to the IC being both closer to the spring eye and the axle's roll axis being closer to horizontal.
That also means less roll steer.

The other thing is that due to the decambered lowered leaf pack, the rear shackle is on a greater angle. This has 2 different (and opposite) effects:
1. it increases the effective spring rate
2. the anti-jacking effect in bump travel will unload the chassis, effectively reducing spring rate.
The overall effect on spring rate will be which of those 2 factors has the greater effect. Which is a mystery to me. But as I can't really do anything about it without affecting other things like ride height, I don't really care. It just is what is is.