I've constantly been asking myself the same battery relocation question. I've decided that I'm either putting it behind the passenger seat (prob. on the floor pan where rear passengers' feet would go... if I had rear seats) or on the "shelf" opposite the jack. In any case it would have to be (in order to comfort me) 1/ behind the driver and 2/ to the right of the driver (offset driver weight).
As for taking weight from rear/front, I'm pretty sure the car would tend to understeer. It's (AFAIK) partially attributable to FF cars being hard to "toss around" without a ridiculously beefy rear sway bar or without putting spare tires/sandbags in the trunk.
This is also why I feel bad for taking out the spare tire, sound deadening (from rear only), rear seats and rear interior panels. I feel like I really have a responsibility to relocate the battery, put in a pair of carbon-kevlar seats and dump the A/C compressor.
Now that you bring up the issue of lighter rear end meaning less grip meaning more oversteer vs. rear being lighter and having less inertia to rotate around... that's a very good question.
I'm 99.9% sure a lighter rear end results in understeer, but now I'm confused as to why, too. Damn you!
