I built my rifle rather heavy as I have Rheumatoid Arthritis through my whole body. I would rather carry a heavy, in this case 12.5 pounds rifle then deal with the recoil of a 10.5 pound gun, which is about right for a 450 # 2 N.E. However weight and the speed of which a gun handles are not total oppisites. Ballance is critical as well as stock design. If you do not believe this pick up a Holland & Holland .577 and try its swing. No I do not want to carry it all day in 110 degree climates but in these times we do not have to.
Probably the biggest reason that most U.S. gunsmiths do not like the tappered to the muzzel barrels is due to the fact that on this side of the pond most of us do not shoot rifles with any degree of recoil. Many people who stop by my shop are reluctant to shoot a .338 as they feel it will kick them out from under their their hat. For the most part up here in Maine a 30-06 is about the upper recoil limit. You have to look at it in this perspective. Most Americans who hunt fire around a box of ammo per year. Few have hunted out side the U.S. and those that have for the most part still fall into the box a year catigory. Recoil is somthing you have to work up to, just like with a child, you do not start them out with a 12 gauge. The light muzzel weight double you mention is great for a seasoned shooter but most over here would find the recoil too much.
I have to put this in though, A properly designed rifle with a bit more weight may be a tad slower to bring up but it will be one hell of a lot faster on that second trigger.
I am no expert but with that rifle of mine I can put 2 bullets into an 8" circle at 50 yards in under 4 seconds on a good day. I do not really know how good that is but I feel sort of pleased with that.