With a 4.1:1 diff and 265/75R16 tyres (2.525m circumference) cruise speed is altered from 94km/h to 89km/h (instead of the 99km/h that those tyres would produce). This would lead to savings in fuel (less strain turning the wheels over). Not a bad thing if you're going to stay on larger tyres.
Even better - if you use 285/70R16s your cruise adjusts from 94km/h (standard 255/70R16 cruising at 2,000rpm) to 92km/h. Makes this diff PERFECT if you go the larger tyre.
If you use 285/85R16 tyres your cruise (@2,000rpm) should be 109km/h - and this diff brings cruise back to 99km/h.
I'd recommend the diffs to anyone wanting those tyres.
Note: it's important to consider the YD25 at 2,000rpm because that's the point where your (stock) engine is producing the most amount of torque (for maintaining speed) for the least amount of fuel. Any lower RPM and the torque falls away, any higher and the fuel input rises faster than the torque, making it not worthwhile.
In the V9X, that's 1700rpm.
Here's the formula if you want to whack it in a spreadsheet and do it yourself:
60 times cruise RPM times tyre circumference in metres divided by (top gear ratio times diff ratio) divided by 1000.
Tyre circumference is 3.141 times (2 times tyre width times profile divided by 100 PLUS rim size times 25.4) - then divide this result by 1,000 (convert from mm to m).
Examples:
255/70R16 has circumference of 3.141*(2*255*70/100 + 16*25.4)/1000 = 2.397m
285/85R16 has circumference of 3.141*(2*285*80/100 + 16*25.4)/1000 = 2.798m