The problem with this line of thinking is that there simply aren't REMOTELY enough examples of hitters that have made the transition to be able to generalize like you are. Suzuki is way better and, more importantly, WAY different than Tsutsugoh, even when Tsutsugoh was in his prime -- simply the fact that they both hit thirty-plus homers is not enough to compare them. It would be like saying Fernando Tatis and Pete Alonso are similar because they both hit thirty-plus homers this year. There's reason to be skeptical, yes, but the only way to project is to use scouting and evaluate the players as individuals -- which maybe we can't do well from where we are, but the conclusion should be uncertainty, not pessimism. Or, put another way, the skepticism should come from our lack of ability to draw a clear conclusion, not from a sloppy conclusion we DID draw from a lack of data.
Also, there are some small parks in Japan (Jingu, Tokyo Dome, Yokohama), but there are just as many cavernous, pitcher friendly stadiums (Sapporo, Chiba, Nagoya). Homerun numbers are not inflated over there at a league-level.
Finally, I think it's important to point out that, contrary to what I think most people believe, the players that get posted by NPB are RARELY the best players in the league. They are invariably GOOD players relative to the league (nearly always past their primes), but NPB teams work hard to keep their superstars. The reason that Seiya is so intriguing because this is the first time since Hideki Matsui that a player is posted that is arguably the best overall hitter in the league. I think you can make a case that Masataka Yoshida and Munetaka Murakami might be better overall hitters or prospects, but it wouldn't a definitive one, and neither of those guys just had the season that Seiya did. I can tell you that, for sure, how Seiya fares will have a major affect on how I evaluate the hitters in Japan for years to come, because we simply haven't been able to see someone like him, in his prime, take a real shot at it... really since Ichiro. Even Matsui was significantly older and clearly out of his athletic prime when he made the jump (he used to play CF, if you can believe it).
It's going to be interesting.