Sounds like you engine can pull all the air it can swallow through a 38mm venturi
On our fully race tuned CR250’s we find the optimum size to be 41.5mm. However, a 38mm will give exactly the same power up to about 9,500 rpm. It’s only really the last 1,000rpm from 10,500 to 11,500 were you begin to see an appreciable difference of about 2 Hp at say 11,200 rpm. Of course if your engine is in a lesser state of tune and doesn’t pull those kind of revs or the power drops off after 9,500 rpm, then your wasting you time, effort and money. Can you post up a dyno graph for us to see?