Can't say I'm familiar with any Sega sound guys named Takada off the top of my head. I checked out the Takadas currently in the database and none of them seem particularly compelling as a Sega arcade composer active during the '80s, but who knows.
Yasuhiro Kawakami shows people can have gotten their start in VGM earlier than anyone thought.
Then again, maybe he's another case like Lotty and was never credited under his real name!
The identity of "Dolphin" has been pretty much been my #1 VGM question since last year, and even though this isn't a complete identification, the ruling out of Nakabayashi is a big development indeed. Sucks that I jumped to that conclusion too hastily, so sorry to everyone who got the wrong info after that association escaped VGMdb and got into the wild. And big thanks to you (dissident93) and Masaru Setsumaru for clearing up some of the mystery!