There's a fellow BPCR longrange shooter that has been firing test loads into a snow bank all winter. Now the snow is retreating and the bullets he's recovered so far aren't showing the wads doing a whole lot to keep blow by from happening.
The load was 68 grs of cartridge dropped from a Lyman 55 measure thru the 24 inch drop tube into remington nickle 45-70 cases , .030 wad, and the RCBS 82084 bullet cast at about 20-1 ,sized .459 and lubed with sagebrush alox. Range was 270 yds on a sort of a miserable day for shooting, overcast , snow on the ground and fishtaling winds 10mph.
One crutial factor in wad performance that I seldom see given in load data is diameter of wad.
From my relatively small amount of test shooting with a big bore BP rifle, I have seen wads be very useful, as well as giving indifferent effect on results (especially on leading). Important parameter seems to have been diameter of 0,060 wads I was using. Too small wads, no surprise, provide little or no additional gas sealing -- and why should they? Wads large enough to fit tightly into cases and to be slightly larger than chamber diameter did greatly reduce leading (reduced powder gas blowby). Total elimination of leading came with combination of pure lead bullet, that could be pressed into unresized case with easy thumb pressure, and one, same diameter 0,060 gasket fiber wad.
Admittedly, rifle used in above described results has oversized chamber and well eroded first 15 cm of bore. Still, with these pure lead bullets and same sized wad, 3-5 shot groups are as good at 100, 200 and 300 meters as I can shoot with any rifle having rather coarse sights intended for use in deep, dark conifer forests when hunting moose where a long shot is 100 meters and most well under that.
Perhaps with newly manufactured rifles of standard chamber and bore dimensions, selection of effective wad is more a matter of choosing a standard diameter wad and then a suitable thickness.