I have those PIAA Superplasma GT-X bulbs too. While they look really great color-wise, they aren't any brighter than stock. I thought they were at first, but the color does fool the eye. I've switched back and forth between them and the stock bulbs to be sure. I don't know how long they last - mine are about 6 months old.
After much research, I ordered the Osram Silverstar from England for my daughter's Lexus. They aren't coated like the Sylvania version sold in the US. The light is pure white and they are a little brighter. They're comparatively cheap too - under $25 shipped from
http://www.powerbulbs.co.uk/ - be sure to use the 10% coupon code - ZXRCX111.
While I was just there, I saw that they are pimping the
Philips X-Treme Power H4's. I wonder if they're as good as the hype.