I've had Cornus mas variegata and Cornus kousa chinensis about the same length of time, I think 6 or 7 years. I don't have the moist soil they prefer, so they have both been very slow for me. The kousa has fruit about an inch, spherical, very showy (this tree really is an all-rounder ornamentally - I really can't think of a better one). The skin is thin but leathery and you can't eat it, but you easily break through to suck out the squishy flesh: you have to spit out a few pips a little larger than grape pips. The flesh is squishy pulp, custard-like, but very sweet and delicious. My tree (still only about 2m) fruited the first time last year, and was quite heavily covered in them, about this time of year I think. But it didn't flower at all this year.

Cornus mas fruit are much smaller, about half an inch long, oval, slightly flattened, with a large hard olive-like stone to which the flesh is well attached. I find they are often very acid, and I don't bother with them very often. Maybe you can get cultivars which are selected for the fruit. The number of fruit is probably similar to kousa, but they are very much smaller. I doubt I'd get half a pint of the tree, but it is still small. The tree is about the same height as the kousa but has not bushed out as much. I've found its flowering time is very variable, it has flowered anything from Jan to April depending upon the winter weather.

I also tried to grow C canadensis, but it died - it needs damper conditions than I can easily give it. My C sibirica also decided to suddenly die this year, I don't know why. It was never any where as near as vigorous as the C alba next to it.

It sounds like your mulberries are Morus rubra rather than M nigra, but (unlike M alba) they are supposed to be just as good to eat. You are correct that M rubra is dioecious, but M nigra is self-fertile. M rubra is not often grown in Britain. It naturally grows in New England, so I imagine it is very hardy.

I agree with growing unusual things when normal things are so cheap in the shops. I've wanted to try Asimina triloba, but it seems very tricky to get hold of the plants, they have to be put in final position very young, and they need cross-pollination, so it all sounded too much like hard work and too much space in my smallish garden for fruit that probably wouldn't ripen in a normal summer here. I had a feijoa but I planted it in 2003 and it expired from drought/heat when we went on holiday at the height of the heatwave. I have a new location against a new south-facing wall I'm thinking of putting another one in, but they are supposed to need cross pollination for fruit.