plavutka

I don't know theory for marine propellers, but since both water and air are fluids, I start from the basis for calculating an airplane propeller:
P=D**** x n*** x h** x A
P = power
D = diameter
n = revolutions
h = pitch
A = propeller face square
There are also some small things like viscosity... but for the comparison this is not important.

By this two examples I considered 6000 rpm for LS376/480 and ZF68A gearboxes:
a.) 15"x22" propeller and gearbox 1,583:1
b.) 15.6"x30" propeller and gearbox 2.04:1

Both times the result was the same power, but in case a.) it was only 80% of the speed from case b.)

Since boath resistance increases with speed, but at the same time there is less slip in b.), I can conclude that in cases b.) the final speed will be at least 4-6 mph higher.

Accelerations from case b.) are incomparable better to a.)

In case with 1.583:1 the LS376/480 engine make less torque on the propeller shaft than the Merc 8.1 HO with 1.5:1 and shaft speed of 3.790 rpm ,
but in case with 2.04:1 it has more torque than the Mercury 500 EFI with 1.5:1 and shaft speed of 2.941 rpm which it is smilarly to BB HO with 1.65:1. (Reggie solution for higher speed 1.5 vs 1.65?)

BB prop speeds:
3.700 rpm has BB (5000rpm) with 1.35:1
3.333 rpm with 1.5:1
3.030 rpm with 1.65

Tartilla

Unless you're using an aluminum LS block, the iron LS bare blocks weigh 200lbs with main caps and bolts.

Bare BBC blocks (Gen V/IV) weigh 220lbs with main caps and bolts.

Obviously the BBC iron heads and cranks weigh more, + the iron intake if it didn't have an aluminum. BBC iron manifolds will be a bit more as well...not sure what the new LS is using.

Everything else is pretty much the same weight for marine dressing.

You've maybe shaved off 200lbs ish.