Do you happen to still have the packaging the bulbs came in? If so, I'd check to see if they're labeled as being 'instant-on' bulbs. The term is a bit misleading as you already stated that the bulbs instantly come on when you flip the switch however, this term is actually supposed to refer to the warm up time. If you get bulbs that are labeled instant-on then you should not notice a difference in brightness from the time you flip the switch to 15 seconds later.
That reminds me of several years ago when one day, out of the blue, I went to the store and bought enough CFLs to replace all the lights in our house. Not until I was finished did I notice a delay from flipping the switch to having it turn on, in addition to the light becoming brighter in the first minute or so it is on. I didn't mind because I was saving about 75% on my lighting costs however when my wife got home that night she was
when she had to wait half a second in every dark room she walked in to.
CFLs have improved greatly since then, but I'm not buying new ones 'til all these suckers bite the dust!