At one time or another, most of us have tried to justify the next big purchase with the IQ argument, usually sharpness. Here in my lightroom I'm looking at a Tamron 15-600mm f/5-6.3 G2, Nikon 200-500mm f/5.6, and Nikon 600mm f/4G because of it. Along the way I came to realize somewhere around 75% of IQ is technique especially when and where under what light to shoot, planning and patience for the right subjects and composition, using the best metering, AF, DOF, and exposure settings, avoiding camera shake, ... , and how much time you spend in the field. Another 15% comes in at post processing where you can have a big impact on composition, exposure, color, and sharpness / detail. The last 10% is your gear, as long as it meets a minimum threshold for quality, and the 200-500mm f/5.6 does that.
Yes, I can see a bit more detail in my keeper images taken with the Nikon 600mm compared to the Tamron G2, but the subject, composition, and story are much more important. So the question I now ask myself is in order to produce better images should I spend my money on more expensive gear (%10 impact), studying PP techniques (%15 impact), or improving my knowledge of photography, shooting technique and spending more time in the field (75% impact)? Only you can answer that question for yourself, but don't get lured into thinking that spending $3600 on a new lens will get you significantly better images and using the feedback on a forum to justify the purchase.
The best advice I got from a pro when I asked about better gear was to rent it first to see how much of a difference it makes.