Steve,
Old bulbs get dimmer as the tungsten burns off the filament and yields a dark tint to the glass. From my reading off the web, the (U.S. version) Sylvania Silverstar is not significantly brighter, but a marketing gimmick, specifically designed to empty your wallet. They have a slight-bluish tint to the glass which gives it the "whiter" color temp so they look more like HID.
If you stick with Sylvania, consider the Xtravision. They are reportedly brighter by up to 30% than stock, and cheaper than the Silverstar, but it lacks the "HID coolness factor," as the bulb does not have a bluish coating. Philips Premium bulb would be an equivalent.
In Europe, Philips' VisionPlus and Osram Silverstar (not the US Sylvania Silverstar version) bulbs are reportedly up to 50% brighter than stock bulbs. Both do not have the bluish coating. (Hint! recurrent theme here: blue filter coating reduces available light output.) I do not believe that these bulbs are US DOT approved, but they should still be safe as they are still stock wattage. These bulbs are easily available on the internet, but I'm not sure if they're available in your size. Personally, I'm going to give these a try. They should be better than the Philips Long-Life that came as stock. (Long life = dimmer, but longer life, so less warranty replacement costs for Lexus.)
Don't have unrealistic expectations though. Improvements in illumination will vary depending on the optics of the headlight. And, stock wattage halogen lamps are not going to have anywhere near the brightness of an HID system. Also, even if there is an actual 30% increase in light output, it does not translate to a 30% increase in perceived brightness, because human eye & brain response is not linear.
P.S. - I'm no expert in this field, but that's my summation of my reading on the topic.
As you may know, it's very difficult to get specs on light output (in lumens) on the bulbs with casual reading. It's probably designed that way so we can't compare apples with apples. I did find useful information on the following web site: http://lighting.mbz.org. Sorry for the long post, but hope it helps!