I've got a pair of the Jamstrait 3157 LED.
I noticed two things.
1) It is brighter, but only in a small pattern. LED's don't have the spread pattern that incandescants do. At night it manages to light up the whole assembly, but not during the day.
2) I'm guessing it's the stepping down of the voltage but the blinker goes into hyper mode like it would if one of your bulbs was out.
I've gone back to incandescant 3157s.