On our trip from Sydney to Townsville I ended up using one of those neck donuts for babies for him to sit on, and we alternated him and his brother in it so they got a break. Otherwise its a perfectly fine seat for everyday use! Esp with the adjustable headrest. Im not entirely sure a cushion wouldnt compromise safety. Then again I think most carseats for the older kids arent cushioned enough on the seat.

ETA - Ginger, Im assuming you'll be using it for your Ds1. Just an fyi that the 5 point does beautifully up to 20kgs. my Ds1 is 5 and 19.5kgs and still fits the 5 point perfectly in the Hi Pod. If you got the 5 point-convertable one.