Well that's a lot of light (and heat) for fixture rated for a 15W lamp. And of course, UVC light is not good for skin and eyes. Here is some more info about UVC lamps.
- Actually, no, the opposite. The lamp will attempt to draw more current from this ballast than it can provide, causing the ballast to get warmer than normal. Meanwhile, the lamp will not produce the full quantity of light it is supposed to, since it is not getting the full power it is rated for.
Furthermore, UVC lamps appear to have some unique qualities which may make them incompatible with traditional fluorescent fixtures. Namely, "[UVC Lamps] generate harmonic and transient voltages and currents. They exhibit a seemingly strange characteristic – the voltage decreases with increasing current. This process is known as negative AC resistance. Therefore, gas discharge devices, such as germicidal lamps, are inherently unstable; once lit, the current increases without limit unless other circuit elements - ballasts - are used to restore and maintain operation."
- As long as a new lamp is a "UVC" type, rated for 15W, it may emit light just fine (but of course, be a "smaller" lamp, and produce less light.) However, if UVC lamp operation is incompatible with the ballast in the fixture (designed for traditional fluorescent lamps) then it might not work at all, or destroy itself, or the ballast.
Note that since UVC is a rather unusual light frequency, some materials may be incompatible with it, such as the plastic cover for a traditional fluorescent lamp. I'm not sure, but UVC light could damage this cover, or not pass through it at all. Note that most UVC fixtures seem to have no covers over the lamps at all. At 46W per lamp, they are going to get a lot warmer than 15W lamps also, so the added airflow may be necessary simply to cool them.
My advice is to get a lamp and fixture designed for the purpose you have in mind. It may be more expensive, but it will be more robust and operate correctly, without fear it may overheat or fail catastrophically.