[1speedbike]

GP thunders will be slightly LESS bright than stock. A lower color temperature (like that of the stock bulbs) is going to produce a slightly less intense light for the same amount of power going in. With the engine on, you will see them light up slightly brighter, but for a direct bulb replacement like the GPs, they will be less bright than stock.

Alternatively, if they look much less bright, you could have not screwed the bulb in all the way when placing it into the headlight opening. If it isn't all the way in, the light won't correctly follow the tract of the corona rings, and they won't appear as well lit.