THAT IS 1/8 STREAM, YOU'RE JUST PRESSING IT WRONG.
170 x 8 = 1360, 250 x 4 = 1000. Marisa's 1/8 streams are faster than 250BPM's 1/4 streams. Now let's take into account HT... that would make Marisa's BPM reduced to 127.5 . 127.5 x 8 = 1020, which is STILL FASTER than 250BPM's 1/4 streams.
Conclusion: you're doing it wrong.
Also just to remind that half-time makes a song go 0.75x slower and not 0.5x slower. Maybe even 0.8x, IDR.
I still do have suspicion on kanopu though. Well, with Rokodo... errr, maybe I'll keep that issue to myself just to avoid commotion. youngdow himself did say that with the TaTaCon you can pretty much do that though (believe it or not, I've tried; indeed you CAN press faster on the drum than on the keyboard, mainly because of the opposite force that goes to your hand when you hit the drum, making movement a bit easier)
To check, maybe I can make a Yawaraka-type Taiko map on
http://osu.ppy.sh/s/9917 , and someone try to clear that with an FC. I know youngdow can do that lol