I've been running Lipos in my X - 6 for about six months now, and the car feels great with them. If anything, you actually get a little more rear weight bias, as you've essentially taken the two cells off the front end (most Lipo's are about the weight of four cells).
The roll of the car (because the battery is now sitting all the way across the chassis) hasn't been an issue, especially as I've moved my GTB (brushless speedo's are actually about the weight of two cells) to the middle of the chassis where the two cells used to sit.
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