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The problem is more likely to e the throttle body, need at least 30mm instead of 28mm .
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The Kawasaki factory Fuel Injected EX-250 model uses 28mm bores in its throttlebody. It seems to me you should easily be able to make a good bit of horsepower above the stock Carbureted or Fuel Injected models by using 28mm throttlebody bores in combination with a high-flow air filter and a good quality aftermarket 2-into-1 exhaust.
A 28mm bore is a very good choice for a 125cc cylinder. Plenty of capacity.
Let's lood at the Kawasaki Ninja 650R in a comparison to prove this point because it's also a parallel twin. The Kawasaki Ninja 650R uses 38mm bores on 325cc cylinders. A 325cc cylinder is 2.6 times the size of the EX-250's 125cc cylinder.
Now, when you compare the Area of a 38mm bore to a 28mm bore you get an Area of 11.34cm squared for the 38mm bore and 6.15cm squared for the 28mm bore. The area of the 38mm bore is only 1.84 times the size of the 28mm bore. This shows that Kawasaki actually gave the Fuel Injected EX-250 a larger (relatively speaking) throttlebody bore than it put on the Ninja 650R.
Of course these two bikes (EX-250 and Ninja 650R) are in Kawasaki's "Sport" line. When you go up to their "Supersport" level the ratio of throttlebody size to cylinder size is absolutely crazy. As an example, the ZX-6R uses 38mm bores on its 150cc cylinders. Obviously this combination works, but it takes some real careful engineering to pull it off. The ZX-6R has a very technically advanced airbox and it has a set of ECU-controlled butterflies in its throttlebody. If it didn't have these features it wouldn't idle and it would stall when you gave it throttle from idle or low RPMs.
If you go up to a 30mm throttlebody bore (or larger) you'll probably find yourself dealing with some less than optimum off-idle throttle response characteristics.
If you look at the throttlebodies that Honda/Suzuki/Yamaha/Kawasaki are using on their supersport bikes you'll see that when you go "oversize" on the throttlebody bores you have to put in a set of ECU-controlled secondary butterflies (although some of the earlier FI bikes actually used vacuum-controlled slides just like you find on carbs).
Really big throttlebody bores (oversized for the cylinder) are great when the engine is at high RPMs, but they're a real problem at idle or coming up out of idle RPM or when you roll on power from low in the engine's RPM range.