I've found that most times a sequence/midi will usually sound best in the key it was recorded/sequenced in. That's not always the case but as a rule of thumb I would say most of the time.
Instruments have a limit or range on the Keyboard where if they go beyond their range they lose their 'luster' so to speak. Too high and some voices will start to sound thin and not give a true representation of the instrument. Too low and some instruments literally bog down sound wise and lose accuracy and definition in their reproduction and can sound "muddy" or lackluster.
Usually a song/sequence is recorded while the person doing the Mixing/Recording produces it to sound the best, ie., best Key/Time Signature, right tempo, best instruments, best overall sound.
So when you transpose that song[s], whether up or down, your departing from that blueprint and the different instruments will be affected accordingly. Sometimes just slightly, sometimes not even noticable at all, but sometimes very noticable.
Sounds like you have a "very noticable" one.
Best regards,
Mike