as I understand things (beware: I am not an expert I may be wrong) the dellorto SHA carb relies on this venturi tube AND the main jet to control fuel air mix (there is also an idle mixture adjustment screw, but thats for idle and fairly self explanatory) according the the guide I linked if your altidue is requiring a richer mix it may be better to add another hole than simply re-jet the carb.
the jet will control your maximum allowable amount of fuel for full throttle, but the holes in the tube meter the gas entering the engine further by using the throttle slide.
for example my SHA had a point in the throttle where the slide uncovered the top hole, at this point the bike would get a sudden increase in power (while accelerating hitting this point felt somewhat like shifting your gears but the bike was a single speed)
this is because the carb is leaning out the mix slightly as I increase throttle until the top hole is uncovered then the mix becomes rich again.
so assuming your engine has enough flow overall to deal, you mentioned that the carb has dropped your power so I am assuming that the engine does have the necessary flow, adding a third hole (or widening the bottom hole, which is a riskier idea you can cover the new hole, if widening doesn't work you need to rebuild your carb to go back) will remove this 'step' in throttle response, and when at full throttle you will have more gas entering the engine, you can think of this as allowing the carb to meter out the amount of fuel you have re-jetted for.
all of this having been said, before you do anything at all.
TAKE THE CARB OFF THE ENGINE AND LOOK INSIDE IT. when you pull the throttle double check that the top hole is uncovered, I managed to assemble mine once where the slide didn't move right, you may have done the same.
I made this bit red because I re-read this post and the super important bit is at the bottom. redness is easier than editing.
