I've made that jump and for my 5 year old I would agree and say there is slightly more room for him. However for my 2 year old who is still in a seat (5yr old uses a booster cushion only), I sadly have to report that there is less room for him. This is not due to the back seats - all the above comments are correct regarding size and depth etc, it's actually due to the sparco seats. They are thicker than the recaro ones so take up more of the area than previous. It's not a massive difference but one I have noticed and had to rectify by keeping the passenger seat more upright than is ideally comfortable for anyone sitting in it. Usually I have to let my son have his legs down the centre if my wife is with us. It's nothing really new to how it was in the S1 just that it is noticeably tighter for my youngest.
Saying that, my youngest loves the 400 (and Lotus actually - see pic wearing his fav t-shirt this Xmas) and in 6 months he'll prob be ok in a booster seat so then he'll be fine.