This simple code overwrites part of the routine that makes samus jump. Uses no free space.
It seems that the original intent of the routine was: if samus has speed booster, add half of her X speed to her Y speed when she jumps.
however, there were two issues in the vanilla code:
1) The code adds half of her X speed, but not half of her X subpixel speed, to her Y speed and Y subpixel speed respectively.
2) After adding to her Y subpixel speed, the carry is discarded before adding to her Y speed in vanilla.
this means that if (samus X subpixel speed) + (samus Y subpixel speed) >= 1 pixel/frame, then her final speed will be 1 pixel/frame less than it should be.
This was often easily noticeable when jumping with speed booster without high jump, and would often result in jumps being lower than jumping without speed booster would have been.

This patch fixes both of these issues.

One last thing to note: On the hex tweaks page, there is a hex tweak which claims to fix this routine. I do not recommend you use that, as it does not fix either of these issues.
As always, if you experience any issues with this patch, please let me know.