1) Taking damage there would be a lot slower;
if you enter a boss battle with just one kong you'll be forced to break a DK barrel.
Also, Dixie is the preferred character for almost all boss fights, so you'll need to switch to her.
2) Those porcupines tricked me on the 100% run, but not anymore;
you can spawn the last ghostly rope before the porcupines! And since rolling through an enemy stops the screen for one frame, it's faster to avoid them.
I've asked a question on BizHawk's bug report, and adelikat suggested me to try the Performance mode.
I did it and the bonus fanfare plays at a normal speed. Now I'll try it on Snes9x 1.51 and I'll post the results here.
On Snes9x 1.51 it's super smooth and the fanfare plays just fine, but the frame skip must be adjusted as follows;
Otherwise it'll play too choppy with automatic frame skip.
(or too fast with auto frame skip turned off)
So, the problem is the BSNES Compatibility mode. But it's kinda late to regret my decision to use the PAL version with BizHawk...