Between the Chinese rules and Japanese rules, under a normal situation (no seki, or pass and get one extra as komaster), where there is a difference before komi, it is always black that gets one more point under area scoring.
If the area score difference (before komi) is 7, it can only be a territory score of 6 (different where black plays the last move) or 7 (the same where white plays the last move). In this situation with 6.5 komi under territory rules, the first case white wins, but the second case black wins, however, under the Chinese rules with 7.5 komi, both cases white wins (hence when white plays the last move, the winner is different). If we add seki, the situation gets complicated since it is now possible to have an odd number of shared seki points, as well as seki eyes counted in one rule, but not the other. In those cases, the difference can add up to more than one point difference.
Then we need to count how many games using the Chinese rules ended up with W+0.5 (which can be W+0.5 in the Japanese rules or B+0.5 in the Japanese rules), and how many games ended up with B+0.5 in Japanese rules (which almost always be W+0.5 in the Chinese rules, only on rare occasions we see even difference with seki or other situations). I think the rate of winning by half a point as black under the Japanese rules is about 2.9% or something, And under the Chinese rules, the game ended in W+0.5 maybe slightly higher say 3%, but maybe about half of them give a different result say 1.5% (white played the last move). Assume games played in Japanese rules (and Korean rules, they basically give the same result) and the Chinese rules are about the same amount, the average rate of seeing a game that would give a different winner would be (2.9+1.5)/2 = 2.2%, which is about 1 in 50 or so. There are some 5000+ pro games played each year (those got recorded, the total is probably close to 10000 if we include all preliminaries), so the number of games that ended up with different winners at pro levels using different rules are at the order of 100 to 200 games a year.
I’ve manually scanned through the records on https://kifudepot.net/ for 500 games, and I found exactly 10 games that ended with different winners using different rules, so this estimation is pretty accurate I think (I didn’t even look for more rare cases like with seki or get last move being a komaster that need to dig deeper with larger winning margin, which definitely brings the ratio higher). Regardless, hundreds of games a year with different winners is definitely not something we can simply just say it doesn’t matter.