replace S1 with S2

I have tried to replace a pair of Xbee series 1 on my application with a pair of Xbee series 2 (latest firmware) to make files transfer with one coordinator and one « end device ». In normal mode, transfer takes about 30% longer (14s with Series 1, 20s with series2) , but if I use the sleep command on my end device (SM=1) transfer is 4x longer (about 20s in normal mode and more than 1mn when I wake up m with the Xbee series 1 devices y end device). I don’t understand why my application works fine with the Xbee series 1 devices (I use the same sofware on my application).
Is it possible to replace series 1 with series 2 without losing performance ?
Thanks in advance.