Good question. I did a little online research on this.
From what I found, the H11 is rated at 55 watts and is typically used for low beam. The H9 is rated at 65 watts, is typically used for high beam and (on wattage only, not to mention beam pattern) so is brighter.
Comments on Amazon said because of it's higher power rating the H9 will not last as long as the H11. One commenter said they used the H9 in place of an H11 (not for fogs?) and had to do some modification in order for the plug to fit. I'm not sure if the car's plug needs to be cut up, or the bulb. I think this is intended to keep from using the wrong bulb.
Here's a link to an ordinary H9 bulb on Amazon: Philips H9 Standard Halogen Replacement Headlight Bulb, 1 Pack. On this page you can also select Philips' H11, and read reviews and comments for both. It's where I found most of my info above. It looks as if you are not the only one to try different bulbs.
You'll have to let us know how it turns out. I think it's worth a try - and a write-up.
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