The boots fit kinda big. But I wear two pairs of boot socks anyway plus a pair of polar fleece wader pants. In my opinion, when you have a little space in your boot, your feet stay warmer. It's trapped air that can warm up. Tight boots can restrict circulation, thus making cold feet.
I've never had problems with my wader seams. I can tell you that after walking in them and putting out dekes, I am kinda wet inside the waders. But thats from sweat, not water leaking in. That tells me that the 5mm thickness is doing it's job. Holding my body heat in and keeping the cold out!