As far as predator hunting goes, the main reason I recommend a power-tune for the Korean guns is to get a flatter trajectory. Even in stock form, I would be amazed if a 200grain slug shot from a SamYang failed to completely penetrate the vitals of a coyote at 50 yards.
I myself haven't shot a critter with a stock SamYang (about 180-200fpe), as my rifle came to me already tuned. I recently got the exhaust valve fixed, and it is now putting out 275-285fpe which is a tad more than what the .50 DAQ (Quackenbush's lower-powered bigbore) is advertised to produce.
Not all "custom" airguns are worthy of the title (some are what I would call "polished turds"), so do a bunch of research on them before buying (particularly accuracy-wise). Make sure the builder is a real machinist.
Keep in mind, that the SamYang 909 is a carbine rifle with a 21.6 inch barrel that weighs 7.5lbs, compared to the majority of custom bigbore airguns that typically have barrels anywhere from 28 to 35 inches long, and are much heavier than the 909.
I'll be getting a Gargoyle (28 inch barrel) in the future, but the 909 will always be my go-to calling rifle for calling coyotes/bobcats in these Oregon rain forests. It's the right tool for the job.
IMO, in the world of custom bigbore airguns the Gargoyle is the King of the Mountain. 600+fpe out of the box with the extra power valve, with three other valves available for varying power levels/boolit weights that are easily interchanged.
https://gargoyleairgun.com/Home_Page.html
Here are the results of the latest round of chrony testing with my tuned 909. If you want multiple shots per fill using the same POA in a handy package, a tuned 909 is hard to beat, and as accurate as any custom .45 bigbore I've seen.
Hornady .457RoundBall
3000psi
886.3=249fpe
874.3=242fpe
859.3=239fpe
Hornady .457RoundBall
3100psi
873.2=242fpe
865.3=237fpe
843.9=226fpe
Hornady .457RoundBall
3200psi
870.4=240fpe
876.6=244fpe
858.1=233fpe
BigLube ROA's 210grains approx BHN9 alloy
3000
767.4=274fpe
773.1=278fpe
764.7=272fpe
744.9=258fpe
BigLube ROA's 210grains approx BHN9 alloy
3100psi
743.3=257fpe
771.2=277fpe
769.5=276fpe
758.7=268fpe
BigLube ROA's 210grains approx BHN9 alloy
3150psi
748.6=261fpe
773.6=279fpe
775.3=280fpe
767.8=274fpe
BigLube ROA's 210grains approx BHN9 alloy
3200psi
750.6=262fpe
780.9=284fpe***
780.8=284fpe***
758.4=268fpe
Sure Fire Raptors (?217? grains?)
3000psi
735
742.6
743.1
Lee ROA's 220grain (from BHD)
3000psi
744.6=270fpe
764.8=285fpe
751.6=276fpe
730
Big Lube PRS 255grain pure lead (edit:actually 259-260grains) .452 (to small)
3000psi
669.8
686.4=271
699.7=283fpe
689.8=274
245grain REAL's from BHD, labeled ".457 un-sized"
3000psi
706.1=270
712.8=276
715.7=278
695.9=263 (this REAL was from a different batch/caster)
Lyman Maxi Hunters 255grains (only had two)
3200psi
673.3=256fpe
696.7=274fpe
693.4=272fpe (this third shot was a .452 PRS 255grain)