6 volts do cause more wear on the servos, that could be a reason. Another factor might be the weight of the 5th cell, however you said you are using a nicad- if you use a 6 volt NIMH it will be close to the weight of the 4.8 volt NICAD... so take your pick. I think they say the weight to power ratio comparing the NICAD and NIMH is 1:3

oh, and here is a link to Radicalrc where he talks about NIHM and NICADS... http://members.aol.com/davthacker/nimhfaq.htm

And here is one about 4 cell vs 5 cell...
http://members.aol.com/davthacker/4or5cellsfaq.htm


hope that helps...