really depends on what bike and how its setup as well as the weight load on the bike.

ive done up to 227km/h with empty Shad SH36 panniers on my z900...no issue...yes it does create some noticeable drag at higher speeds, but ive never noticed any noticeable lift of the front.

fully loaded, ive only done up to about 190km/h iirc.

for most nakeds, despite the "upright" position, ur in truth still leaning forward a little... so ur weight will be pushing the front down. plus. at higher speeds (especially 160kmh above), ur definitely gonna be tucking in coz of the wind. so more weight up front. ur gonna balance it out urself.