I pretty much have the KTM twin to your bike - a 2019 300XC. After fiddling with the stock Mikuni carb for a while, I went with a Lectron Billetron carb, too. After getting the needle height spot on for idle and off idle transition right, I noticed after some trail riding at 1/4-1/3 throttle that return to idle was a little hanging and slow, indicative of a touch lean. It ran great everywhere else across the throttle range. I contacted Lectron about this little lean issue, and they mentioned a trade up program to their Pro Series Billetron. The Pro Series has an adjustable torque jet that can add a little extra fuel at about 1/4 throttle which solved my issue with just a 1/2 turn out on it. The trade up program offered a $600 trade in on the original Billetron toward their $700 Pro Series.
I run a Gnarly pipe, red PV spring, and the PV governor dolly screw flush with the housing. James Dean of JD Jetting has always found the most linear power delivery from a 300 somewhere between flush and 1 turn in on the dolly screw. Factory is often about 1-1/2 turns in. When I get around to doing a top end in about 20 more hours, I'll go with an aftermarket head to tighten up the loose factory squish band but not increase compression. I had shaved the stock head on an earlier 2004 300, and it was the best bang for the buck performance and driveability gain by far. I did have to mix a little high octane fuel to avoid some mild pipe bang above 65 degrees on that one.