Woodleigh introduced the cap for better aerodynamic flight I believe. i believe it is designed to blow away easily on impact. Whether it interferes with the "hydro" or "deadly" "bubble" I have no idea. I question the "bubbles" existence in the first place, in the absence of any real scientific study and evidence. People do like them, and think they work well. Maybe some of them can comment on your question.

I think the main advantage for the expensive Hydro bullet, ie as a reloading component, or in factory ammo, is it is monometal, and so can be used in ridiculous legal jurisdictions which completely ban any lead or lead core bullets. Such as California and some German states.

The Hydro also has raised driving bands allowing the monometal bullet to be raised relatively safely in rifles such as double rifles, which might otherwise have some issues with pressure, or barrel expansion (OSR) in the absence of driving bands.

Back to the .416 Rigby No. 2 Flanged. People have expressed a preference for the vintage .450 Nitro Express as a chambering. And why not? It is still one of the best choices, more than a hundred years later. But if a .416 Rimmed Rigby rifle was regulated for say a 450 gr Woodleigh RNSP and FMJs in .416 calibre, how effective would such a bullet be against a traditional 480 gr .458 projectile in a .450 NE at about 2100 fps. Better sectional density, and probably increased penetrative ability. How fast could a 450 gr projectile be driven by a .416 No. 2? If a 400 gr is about 2400 fps, my guess is around 2100 to 2200 fps.

A negative might be somewhat increased pressure, and a somewhat decreased frontal calibre area, .416 compared to .458. Frontal area is a factor in killing and knockdown power, as well as sectional density/penetration, weight, and velocity.

Curl, as you know, the .450 NE is a favourite of mine as well, in the .450 No.2 NE cartridge. I think a .416 Rigby No.2 chambered rifle, would largely do a similar job with a 450 gr bullet, but lesser job with a 400 gr bullet. Why not one of each? You will have to wait for the one they can send me, before you get your testing example. I think we will both be waiting a long time mate.