I can't speak for others, but my reference to "overkill" is related strictly to cost...
If a different weapon platform and caliber can achieve the same objective for far less cost- not to mention the size and weight of the platform as relates to mobility- than that would be "overkill".
My point wasn't related to energy, in a target application it would be irrelevant.
Far cheaper to get your hands on- and shoot- a .338 LM capable of sub-minute accuracy than a .50.
Here's a plot of two 15-shot groups; both just under 5 inches. .30-.338 Keele with two bullet weights. First fired a 190 then a 200 then a 190 alternately once every 20 to 25 seconds. X-ring's 10 inches.
Bart, I'm not saying its not possible or that you cannot do it. I'm also not saying I am more knowledgeable in the overall gun field or a better shooter than you. It does not sound like you have alot of experience shooting at those distances, but that most of your experience is shooting 1000 or under. The 338lm is NOT meant for 1000 yards. The 338lm is not meant to replace the 300 win mag. Using a 338lm at 1000 yards is as wise as using a 308 at 100 yards. You can do it, but there are much better tools for the job.
What I am saying, is that I do have experience in the mile+ world of shooting, and know many that have exponentially more experience than me. Everyone I know would agree that while the 300 win mag can make it to a mile or even more, it will not be more accurate than a 338lm at that distance, because the environmental factors effect the bullets more. That 1000 yard 1/2 moa group, will open up to more than 1/2 moa by 1500. Not because of the inherent accuracy of the cartridge/rifle/shooter which dictate the linear dispersion, but because of environmental factors like wind. A cartridge like the 338lm fights those factors better than most other cartridges, including the 300 win mag.