Whether you like or dislike the flex is up to the rider. As long as you don't break the part, then it shouldn't significantly affect the steering.

If you climb out of the saddle a lot, and pull hard on the bars, then the flex could potentially reduce your power & efficiency. But, it shouldn't negatively impact normal riding.

What does your mast look like? I have a Bike Friday Gooseneck. But, the stem part appears to be a straight tube. If that is the case, you may be able to purchase simple 3Al/2.5V titanium alloy tubing. What thickness? It may be cheaper than purchasing a stem.