I look at this theoretical issue the other way round.
How much better IQ has it got than previous generation lenses did not have?
Build wise it does not have a Kensington lock and only one row of lens fn buttons - but I think it gives my 400 f4.5 a good run for build quality.
Whether the quick release collar as distinct from the quick release foot than the type on the 70 – 200 S will be better - time will tell.
Some test sites indicate a lens like the 8 times more expensive 400 f2.8 S has the best IQ available from any focal length, with the 9 times more expensive 600 S being not far behind.
I have yet to finish my initial usage of the 180-800; yet gain the impression it is capable of enough IQ to make a high quality A2 print with fine detail suitable for viewing from 15 inches
Turning your question round, asking how little IQ do you lose having regard to the huge price saving might IMO be more relevant.
If you can afford and choose only to use "the best of the best" then paying an extra $25,000 to carry the two fast (so far) 400 and 600 primes weighing an extra 10 pounds is a physical challenge, though you can shoot action more easily in lower light, maybe even make a good A1 print at 15 inch viewing distance - and you do not need to consider buying a separate 1.4 TC.
As Sony 200-600 users have known for some time optical development has progressed at a fast rate in this lens range, you get a lot more IQ for your money than 5-10 years ago - and now in the second year of my ninth decade - I can still easily carry the 180-600 on a Z8 or Z9.
A downside - my wife has been working out what percentage of the 180-600 price will buy her a pair of new shoes.