I've also noticed lag spikes with excessive shader usage. It's probably your shader usage that's the issue, 1500 objects should present no problem to PC's today.
Have you tried putting all the shadows on another layer and adding a layer effect? That might improve performance, but it might not.
Your only other option is to manually recolor all the sprites and have a separate shadow object/animation frame.
Otherwise, it's always possible the lag spikes are something 
Ashley can solve. Again, probably not.
Sorry if this wasn't as helpful as you hoped :/