Do you all think a 5 cell 900mah flat pack will perform well in my losi lst? thanx.

It will, but it won't last long. Try to find something up in the 1500-1700mah range. I've had good luck with the flat pacs Trinity used to make. Not sure if they're stilloffering them, but I had a 1200mah yrs ago, and its stilll kicking.