No worries, none taken. As to growth, I have no idea on what the growth rate of a mata mata is. Discus require high quality food, and a lot of water changes, in order to get good growth. Extra filtration is great, but keeping the filter media clean, and the water clean, will be key to the growth of your discus.
And I am in no way attempting to discourage you from keeping a mata mata, they look like an awesome species of turtle. But keep in mind that what sometimes works in one tank, with one turtle, will not always be the same in another tank, with a different turtle. For every turtle that works with fish long term, I'll show you another that has not.
The problem with discus is that all things being equal, size/age of fish etc, they are not aggressive eaters, many people will not even attempt to keep angels with discus for this reason.
http://forum.simplydiscus.com/showt...ish-and-discus-together-Advice-recommendation!
Silver dollars would be a better fit with this turtle due to their speed, and as Chris noted their deep body. (won't easily fit in a mata's mouth) I know that your heart is set on discus, but a school of adult red hooks is pretty impressive as well, without all the drama of a discus.
But attempting to grow out discus, with angels, and SD's, AND a turtle, is a recipe for disaster for discus. The folks on simplydiscus would probably ban Chris from their forum for even suggesting such a mix. lol (sorry Chris!)
I really don't know what the answer is, discus can be uber finicky and sketchy, and they require superb water quality. Discus aren't really messy eaters, unless one is feeding messy foods, such as beef heart etc. In which case you are probably best just siphoning out the debris every day or two while they are growing out. 28C is plenty good for growing out discus, although I know that many crank their heaters up higher. Just remember the warmer the water, the more various pathogenetic bacteria thrive.