Unless his tune is ****, I'll call BS on that. I know for a fact what a 6.0 LS with the work you listed will do and it's well above a day and night difference over a stock TBI or Vortec, hell even a modified version of either of those motor platforms.
Like it or not there is no denying the LS platform is more efficient and more powerful along with better tunability and driveability as well. Only down side I can think of VS old 350s is the parts costs and learning curve of the modern technology for the older crowd.
Tune is good on it! Shifts at 6,500 rpm in Drive too. Makes 315 RWTQ @ 4,500 rpm and 345 RWHP @ 6,400 through an unlocked 4L80E. I still get him 0-60 in my 3 ton brick with a 350. 32" tall tires and 5.13 gears on mine.
I get better mileage than he does, FWIW. 0411 has the same tunability as the LS.
TBI tuneability is also GREAT if you know what you are doing. With my Moates Autoprom I can change every last part of the tune with the engine running if I feel like it.
Even with a 200cc intake port my old 350 makes peak torque 800 rpm sooner and is still making power in the 6,000-6,200 rpm range. Down in the 1,500-2,500 rpm range old 350 is 30-40 ft/lbs stronger than his 6.0L. I run the stock GM B82 converter that stalls about 2,500 rpm behind the 5.7.
Also for the record his truck runs what my crew cab Titan ran with bolt ons and tuning.