You don't need the 38s on a stock motor. If you have larger, higher compression pistons, then you might need the 38 carbs. I had a 89 gsxr with 1216 kit, 13.5 0 1, I used the stock carbs with stage 3 jet kit and ran 9.60 et at 135 mph. My Bandit is a 1246 cc, and I use 38 mm flat slides.
But if you pick up a set of 38s, cheap, it would be a good test just to put them on and see how they run.