Specifics as to exactly how much heat can be removed are dependent on too many factors for there to be any reliable specs like you are asking for. The hotter the component being cooled and/or the colder the ambient air (ie: resulting in a higher Delta T - the difference between room temp and coolant temp) will make the unit more efficient than if there is a lower Delta T, just as will if you have more airflow (faster or better fans) through the rads, etc.
What it all boils down to is there is no specific amount of heat any water cooled system can remove without factoring in other specific conditions which are usually variable.
When you say "water cooling systems" I assume you are asking about AIO cooling units like a Corsair's H105 or H110, NZXT's Kraken X40 or X60, or Swiftech's H220, etc. If that's the case the best you are going to find will some of the comparative testing reviews for AIO coolers. You won't find anything showing exactly "how much heat flux they can remove" but you should be able to see how well they performed in a test system and how they compare to one another.
But I suppose you may be wanting to compare custom loops also, which if so would make trying to calculate even more difficult.
All that said, why exactly are you wanting/needing such info? There's probably no need to make it all that complicated. If you were more specific in what it is you are wanting to accomplish using that info perhaps someone here can better point you in the right direction.