AFAIK the plastic battery box defines the absolute limit in battery size. Don't know about capacity but I see the Rx450 has a much bigger battery. If you look at someone like
Halfrauds, their catalogue items [2] for the Rx400 have one battery that doesn't even fit either width or length-wise! The smaller battery has the terminals on the wrong side
compared to the OEM Lexus version.
I carry a backup battery device in the boot. My 'Man' at Lexus told me to only use the key when intending to drive off, avoid the Aux position [for sat nav and radio] for long
periods and no DVDs while parked and don't leave interior lights on either. I am putting LED interior lights in my 2005 Rx to reduce battery drain.