I have a chassis without battery slots and the difference is not really noticable. Once you put on the lipo tray and the aluminum center brace, the chassis is pretty stiff.

If you need more flex in your chassis, back off the screws in your transmission cases and other plastic parts on the chassis on the top and the bottom plates a 1/4 turn from fully tight, back off the center brace screws 1/8th from fully tight, you'lll need to use locktight on them and let it cure overnight before driving the car.