Not all... maybe even most, led fixture suppliers offer a PAR map for you to see. Some do, most don't, especially with inexpensive Chinese made fixtures (and even some higher priced fixtures from US companies). So how do you judge if a fixture produces enough PAR? You are left to consider other criteria, like led wattage, how hard the led is driven, and how many leds are in the fixture. All I'm saying is that in my experience, fixtures with 3w leds usually produce more PAR at 24" deep than fixtures with 0.3 or 0.5w leds.
I have a PAR meter and do a lot of checking of PAR levels for people in our local club because not many want to spend $300 for a decent PAR meter you rarely use. I've found that even cheap Chinese fixtures with 3w leds run at 2.2 watts of power can create 200 PAR at 24" deep in a tank and cover a 2'x2' area. On the other hand, I have found that Marineland and Current, who drive leds at 0.3 and 0.5 watts don't penetrate 24" with that kind of PAR. They try and make up for it by using a lot more leds and they make good PAR at 12", but it falls off pretty dramatically from there on down.
It was always my opinion, but then I was referred to one of their websites and they showed a PAR map and they even showed that they don't produce anything remotely close to 200 PAR at 24".
I'm sorry, but a PAR of 5 to 20 at 24" isn't enough for even zoas or softies to be healthy and grow. Even the 20 to 40 PAR at 12" to 18" is kind of iffy for some of the soft corals we keep.
You are right about the difference in PAR output between cheap 3w leds and high end 3w leds. That difference is real, but relatively small. The difference in output between any 3w led and any 0.5w led is much bigger. Even with 2 or 3 times as many leds, the 0.5w leds don't get PAR 24" deep anywhere close to as well as a fixture with half as many 3w leds (even when driven at only 2.2 watts).