My girl Pursley will only poo in a clean tray, so if Missy needs to use a tray you've not noticed already contains one of her presents, this may explain why she's using the floor.
I've tried a few things:
Get a second tray and place it in another part of the house (out of smell range)
Change the litter you use for an odour control clumping one (I like world's best cat litter - it lasts ages)
If she doesn't like the litter, try newspapers for a while. Not met a cat yet that can resist newspaper.
I think the Go-cat indoor food has appetite enhancers in it, when I've bought this it goes in a flash, they eat it like sweets, so it's not cost effective for our household. This may explain the quantities she's putting out. I prefer a high protein dry food (Applaws is favourite at the mo) which they eat smaller quantities of. Their poo smells less and there's less of it.
I also top my lot up with wet pouches of whatever's on offer from the Felix/whiskas/Sheba range to ensure they get some moist food in their bellies, and they all like cat formula milk for breakfast
Here's the Go-Cat feeding guide from their website, how close are your portions?
I also note the indoor food contains chicory to aid digestion - just a thought that Missy may have some intolerance to chicory or it just works a little too well
http://www.go-cat.co.uk/Nutrition/Guidelines/