The System Diagnostic tool seems quite accurate from my experience with using dynaudio speakers with 93-94 dB sensitivity. I wish I had something like this 28 years ago when I built my V5 monitor system. AND, it really can pay in pure pleasure to invest in very high clean power if you want 'most live realistic' sound, if your speakers can handle the peaks.
Here's my experience with power and speaker sensitivity. Personally couldn't afford the high end stuff, so I designed and built my own bi-amped two way systems and later added subs in version 5 after 25 years of building different custom systems.
Description of version 5 system:
Stereo bi-amped speakers with dynaudio 8" and soft dome tweeters each capable of handling 1000 watt musical peaks. These driven directly connected by amplifiers (bolted to back of 8" sealed type enclosures) of 450 watt RMS for each 8" and 250 watt RMS for each tweeter. Amps were connected directly to electronic 18dB/octave 3 pole butterworth crossover frequency of 2500 Hz. 8" speakers mounted in ceramic sealed enclosures, and tweeter mounted in 15 pound cement block on top of 8" enclosure and time-aligned.
Sub woofers were 12" dynaudio types, used 4 of these each inside sealed ceramic enclosure weighing -125 pounds each; NOTE: ceramic is absolutely rigid as NOT to flex with intense internal pressure. Subs were driven by stereo 2250 watt per channel custom amp (as were all the speaker amps were custom)
Sub amps were driven by special 100 Hz crossover, with variable bass equalizer so power increased EXACTLY as 12 dB/octave low frequency output of subs decreased resulting in 'flat to 15 cycle' system response. This really made the largest pipe organ recordings come alive; like you could feel the peddle pressure changes in some recordings. It also made these speaker's voice coils vulnerable to burning out, but that's another story
TOTAL SYSTEM POWER: 4500+2x450+2x250 = ~6000 RMS electrical watts = >125 dB SPL ACOUSTIC OUTPUT(measured at 6 feet). System proved it could play all day at near this maximum level without failures, or causing deafness (read below)!
Output was so clean at 120+ dB SPL output that some listeners asked if it could be played any louder! Only when they asked, they could NOT hear themselves speaking, or any reply because the music was VERY, VERY, VERY LOUD, but so very clean they couldn't hear ANY of the usual distortion indicating that a system was 'REALLY CRANKED UP TO BE VERY LOUD.'
Many have commented that after going to practically all the high-end audiophile shows and CES events for decades, they have NEVER experienced anything nearly so clean and powerful like this home built system
The point is, the System Diagnostic is a useful tool to better know that investing in power, and the speakers that can cleanly handle the musical energy peaks CAN BE WELL WORTH THE INVESTMENT for the return in pure listening audio fun in your own home-space that equals or exceeds the live music experience.