Nacho,

The only way the high speed needle can affect the midrange, is if it passes wind (air leak...).
The entire 0-75% throttle range, is up to the low speed needle's mixture curve. The tapered needle is pulled out of the spray-bar, as the throttle is advanced, so tailoring the mixture curve.

If your engine is too rich in the midrange, the needle may be worn, or wrong.
Sorry, I don't know where to find a replacement, or an aftermarket needle. Ask Bill Baxter.

Sincerely,