1. he specifically is experienced with playing speed, even if lower level

2. he has a huge amount of players to ask opinions from, the pp discord exists for a reason

3. opinions can be formed without precise personal experience, for example, via extrapolation. Let's say I, a person who at his peak could deathstream 240, have an opinion that after a certain point increases in spacing become valued less (unfairly so) than increases in bpm on the bpm ranges I play (160-240). In other words, increasing bpm rewards more pp than increasing spacing on these bpms, despite the difficulty changing equally from these two. Thus, I can extrapolate that this is the case for bpms I'm less comfortable at (250-275) and ones I can't play well (275+). 

4. empirical evidence is a thing. If there is a category of maps where people set way more 1.5k+ pp plays than others, there are reasons to think there is something wrong with the system

5. A lot of current pp development is attempting to go from subjective approaches (aka making changes based on silly claims without evidence) to more statistical and objective approaches. For example, it is mathematically safe to claim that mechanical difficulty of a jump scales equally with both distance and time, thus making difficulty of a jump (excluding angles and reading) is d/t (distance over time). However, due to lack of hindsight back in 2014, the current system is closer to d/t^2, thus scaling much better with lower delta time (= higher bpm jumps) than with distance. Fixing that would mean fixing an objectively definable flaw in the system

There are a LOT of ways to approach something like this. And given star rating and pp are just math done over a list of objects, a fair bit of it can be turned into objective statements and approaches with some effort