Brizzy. Did it shift through first second better than new? Mines only 12k ks ridden paperwork from new first owner was complaining about lower gear clunky shifting. Which was still the case when i got it at 9k ks then when we added power 2nd gear problems arose now is worse
I've done a complete rebuild of all output gears and 3/4 on input side...the biggest single improvement came from undercutting all the gears. There are plenty of 250+ bikes running stock boxes with no dramas so it might pay to have a good look at your clutch disengagement too? 12k is pretty early for worn dogs unless you're into clutchless shifts
@warp9.9 might still have some pics for you to look at