You have more experience than me with UV's. I did my research, trusted the advice, and followed it. If the bulb stays clean then you shouldn't have a problem.
As far as lifespan of the bulb goes. A UV bulb does not burn out for several years, it just puts out less and less UV light until then. You really can't check it visually since Light in the UV spectrum is not visible and the bulb is in a sealed container. The bright blue color of the bulb is more of a byproduct of the UV bulb and really just tells you that it's on. A UV sterilizer is for preventative purposes and is not needed for the daily health of the aquarium. Your aquarium is healthy enough that the UV could even be off and you'd never notice from the results. Your bulb if pretty high powered. I would think a 25W bulb can degrade quite a bit before it stops being useful.