The inherent problem is that performance points are driven by star rating, and star rating is driven quite heavily by density of maps. Because of this, 7K maps are going to inherently have higher star ratings, despite being objectively easier for players that are proficient in both games. Because there's three extra lanes, there's much more room to evenly distribute notes in a way that is not quite as straining. For a player who can presumably play 4K as well as 7K, a 7K map will be the easier one to full combo than a 4K one at the same star rating.
I've been personally working on the schematic for a project using the osu! API to develop a site that would basically filter out performance points based on key-mode to give players their "true performance points" in a given key-mode, as there are certain instances where weighting would cause all sorts of discrepancies. I basically looked at
jakads' scores and used it as the basis for why this needed to happen, taking note that his AiAe/Speedcore 300 (best 4k scores, at the time) were weighted substantially lower than they should have been (Speedcore was <35%, whereas in a 4K filter it'd have been 95%).
It's actually getting a little worrisome seeing all of these 7k players coming in because there are a lot of 4k only players whose ranks are suffering a bit, and I'd feel inclined to not care so much, but... when you have a rank requirement to get into the 4K MWC, it's a little rougher on those who are basically just fighting to get above that 5k rank cutoff. I know that I'm not affected, but my ranks have been slowly falling further and further down. In the last MWC, I entered with a rank that was in the upper 500s, and this time, I'll be doing it in the 800s or so (though I've yet to play Blastix Riotz, so that'll help a bit).
