Your math is not quite right. A mAh is not energy, so you don't just multiply the number of cells by the mAh rating. It is the product of current (milliamperes) and time (hours). If I have two cells in parallel, then I get double the current, and thus double the mAh. If the two cells are in series, the mAh rating does not change because the current does not change; the current is simply delivered at twice the voltage.
The original pack consists of 3500mAh or 4000mAh cells. So, using 1500mAh cells in a purely series combination would give you just 1500mAh, and hence shorter, not longer, runtime.