Not sure what 'incoming emissions standard' refer to. Tier 4 Final is just that. The final mandate that causes manufacturers pain and end users grief.
Until the technology catches up to the emissions hardware there will be grief for the end user. Once technology catches up and surpasses the mandate, diesel engines will become like modern gasoline engines. Stone reliable, power producers with no emissions.
Until then, the end user gets to bear the 'issues' of the mandate.
Far as the SAE standard for developed horsepower, that all depends on who is doing the testing. I kind of go by the Nebraska tests when it comes to drawbar pull and developed pto power, though not always. Both my tractors are rated at 80 pto horsepower but my seat of the pants measurement tells me it's more than that.
I go by 2 standards, torque rise, which is the ability for the engine to assume a load without laboring and the amount and quantity of black smoke that comes out the exhaust stack. Most times, my exhaust is clean but when I really put a load on them, like heavy mowing wit the bat wing, I can make them smoke which to me is a good indicator of how hard the engine is working. My engines are turbocharged so the turbo adds to the 'felt torque rise as it provides a positive intake pressure and crams more charge air into the cylinders. More combustion air equals more torque and horsepower, at last to the point where the wastegate opens and controls the intake charge from over boosting the engine.
With a Tier 4 final engine, there is no smoke because the 'smoke' which is soot, is contained in the DPF and gets cremated during regeneration and eventually has to be removed (when the DPF becomes full of cremated soot). Consequently, the 'seat of the pants', torque rise comes into play.