Well, having also used this camera for more than a year, I never used ISO200 or higher, anyway 100 should be perfect for a sunny day like that, and you don't even have to worry about the depth of field with a small sensor.
I've just had a look at the originals and 200 already seems to be grainy.
However, a friend of mine fucked up an entire shoot (those open doors @ the local airforce base) because of a cheap (read : "no need to sell your kidney for those pro ones") UV filter; the fact that you use it with the polarizer could explain a drop of quality. This guy ended up with grainy and out of focus shots with perfect light conditions and low ISO settings.
I'm not sure though, so my advice would be to try without the UV filter and compare the results.
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