In one of the better known studies, Lennard Zinn did extensive testing using a wide range of riders of varying heights, leg lengths and riding styles and cranks from 160 mm to over 200 mm long. His findings said there was no correlation with crank length and power output. Some riders expressed a preference for one length or another but there was no objective improvement from their use.
Short answer: use what you like but don't expect any significant improvement in power or speed. Your recent results may have been because you were expecting a difference and the outcome was self-fulfilling. I don't think it will last long term.