I've used both 5oz and 2.5oz insulation in the SE. Unfortunately, I haven't used the 5oz during the summer, just late fall and winter.
My current quilt quiver is a 2.5oz open quilt and a Golite Ultra 20. I have so far liked having the summer weight quilt and the down quilt for when it drops into the 30's and below. I like having the summer weight quilt mostly because a lot of my summer trips turn out to be overnight SUL trips, and I like to drop as much weight as possible. During the school year, I normally take the time to get full weekend trips to help recoup from everything, and I care less about weight.
With that said, I used my 2.5oz quilt a lot last summer and sometimes when I was in a bivy wth it, I was too hot, at least at first. I think with the 5oz, a well ventilated area, and an open footbox, you would stay comfortable. Hang a leg out and maybe an arm. Don't wear many layers while sleeping. All those small factors that can help you stay cool.
I've converted my old 5oz quilt into an underquilt and will be testing that here in SC over the next week or so. I can tell you how that does. Kinda similar, but not the same for sure.
I would say that if money is an issue, and all your camping is 3 season (not going much below 30*), go with the 5oz quilt, and just have one quilt. If you go out when it gets below the capabilities of that 5oz quilt though, I like the set up I have. With nice layering, that 5oz quilt can go to the mid 20's though, in my experience.
Also, money is an issue for me on gear as well. My guide I wrote on making the quilts is based around saving money. I try to reduce wasted material and reduce shipping costs mostly. Feel free to check it out and ask me any questions on it or give comments.