4.15v would provide you a decent amount of additional power, remember that these are normally charged to 4.2v/cell. So 4.15v/cell is still conservative.
3.6v is about where they are ready to start diving for 0v.. i usually stop at 3.45v/cell, anything lower than that, and the internal resistance rises to the point where the cells can actually disbalance themselves & start heating up.
( we're talking about RC Lipo ( lico ), i take it )
^-- turnigy 20S 5AH pack being discharged.