Acer palmatum 'Fireglow'. Very popular selection for warmer, southern gardens and fits the rest of your specs.

'Moonfire' is an alternate selection but will get somewhat larger with time.

Here is a link that might be useful: Acer palmatum 'Fireglow'

Thanks. Do you know how it compares to emperor I?

'Emperor One' should be quite similar to either of the above although there are some reports of it "greening out" in the heat of summer. Personally, I find the coloring of 'Fireglow' a bit more intense and glowing but it all comes down to a matter of opinion :-) And which you can easily find at the location and size you want.

Sherwood Flame. Got the usual red/purple leaves, then in Fall turns a really really beautiful strong red.
Don't know if it likes heat. Mine is in a shaded position, but no other red I have matches it for Fall color.
Can't remember where I got the tree or how available it might be.

I was looking for the same thing (specs + zone 5 hardy)...I thought Emperor I was the choice selection for holding its color.

I ended up leaning towards a Korean Maple but still considering a red colored JM.

Thanks for the suggestions.
I had ordered before deeproots replied but it was a tough decision. I added Fireglow to my cart then really struggled with the choice of that or Emperor 1. That morning I saw some emperor 1's at a local nsy that were as intense as I could hope for. (Only too big and expensive) It also looks like I had a little more room than originally thought and supposedly the Emperor 1's grow a little bit faster. So I took the fireglow out of my cart and replaced it with Emperor 1. I really wanted to go with something more unique but it should do the trick.

One of my neighbs has a Japanese maple growing in full sun and it seems to do ok and this one would be in a protected location so I think it'll be OK heat-wise.

The more deeply cut leaves of E1 was also helped with the decision.

I would recommend fireglow. Plant it between your window and the setting sun for an incredible show.

My Boskoop Glory has intense fall color, and leaves emerge red in spring if I remember correctly.
It faces west in my front lawn and does very well in the brutal summer full sun. The leaves green up quite a bit by August due to the strong sun, but the fall color is exactly what I wanted - blazing red-orange!!

Thanks for the continued replies however this is an old post and have selected and planted the tree already.