I have one bitti boo but she hasn't done a poo in it yet. The sio and aio's haven't managed to contain the poo though despite her having chunky thighs! Although she poos a lot in one go so can't imagine anything containing it at the moment!
As far as itti SiOs go my LO has chubby legs so I've never had a problem with explosive poo and the ittis.
I haven't used the itti fitteds, but I have some old old old Totsbots I use on her with barely any elastic left. There's plenty of leg gaping, but I just use a Nature Babies wrap which is fabulous! Two rows of snaps and excellent sizing. With the well fitting wrap I can stick anything absorbant in there! I think with fitteds the wrap is more important and the inner bit.
our bitti boo (we have 2) contained a huge bf pooplosion, to my great surprise, more than once! and it isn't even particularily snug fitting as it is a large. not a speck got on the wrap. they also dry over night almost, and can double as a night nappy if need be (with some boosting). i'd happily just use them as our fitteds but I already have loads of other ones. and they stay remarkably soft! i have to stop myself from buying them when I see them.
Again the Boos weren't around when Rufus was tiny but got in great with small AIO's ... So much so I bought loads of M which didn't fit him at all and leaked with the 1st wee. I think ittis are very much a fit or don't nappy as they aren't very adjustable.
One thing I wished I'd discovered earlier though is the Thirsties Duo Wraps, tried every wrap under the sun but these are amazing! Had the size 1 quite late so were quickly outgrown but have used Sz2 ever since