Something to consider, the thermal value of 3 1/2 inch section of a 2x4 is about R4, reg foam is about R5 per inch, so if you cover the 1 1/2" edge of the 2x4 with 1" of foam you are doubling its R value. The quality of the work installing the insulation goes a long way.

I've heard/read that the framing of a structure negatively affects thermal and sound transmission. I've been thinking that for the horizontal surfaces, I'll be replacing existing insulation with much more, new insulation. For the vertical surfaces (walls) that separate the attic from the upstairs rooms, I'll make sure that the existing insulation is in place (there's a few places where it's sagging or falling down) and then attach rigid foam to the back of the 2x4s, encasing the existing f/g bat and the 2x4s. That way I'm keeping the existing insulation that seems to be in pretty good shape, and adding to it, but also insulating the timbers as well.

I think a three inch rigid foam will be good enough. . If there's no holes, like can lights, with painted drywall, there's no real leak according to my engineer. Our warm climate here as winters aren't cold is a huge plus.

There are many homes built like yours here in socal. You will be surprise at the reduction in temp. just by installation the rigid foam during the hot months. I often try and put in a fan to mechanically move the stagnate hot air at or near the ridge or highest point. My favorite is the Fantac remotely or attic mounted bath fan. They are so quiet that they are known to burn itself out because my clients forget to shut it off as it run for months none stop. Electronic timer is my friend. If there's a way to move that hot air out of there, do it. That will outperform any insulation during the hot months during summers.

With 3" foam fitted tightly, drywall can go right over that without additional more to the underside of the rafter. The R value from rigid foam is amazing compared to the "Pink" insulation. Small bottles of spray foam can be used to fill gaps or hold the rigid foam in place.

I am not sure how many sq' of ceiling you are doing, but try the rigid foam and see the difference for yourself. With drywall in place, it will seal up even better.

Just under 800 sq ft,,,, and thanks everyone for indulging me in Steve's thread.

If you can stuff 3.5" of rigid foam up there in between the rafters, that's almost value of R 20. looks like you have 5.5" there or are those 4x8? Having just 4-5" up there is a LOT better then have none. Last I remembered,it was about 3-4 bucks per foot for 4". Good luck and I am sure Steve doesn't mind

Nope, not at all.

The other major source of heat is sun through the windows, quite sure you guy's down south usually pay attention to this, looking at Sat's pic with a black window shade on it. We have a couple fairly large windows facing east and they are triple glaze and uv coated forget what percentage, but the sun entering and warming the floor and room was noticeable, new ICF build (actually still finishing. LOL) so no heat entering through walls or roof. I just ordered some white curtains as a quick fix for the summer months, eventually will do some sort of roller blinds. Just walk around with a IR thermometer and glance at surface temps.

You want to ensure each rafter has ventilation to the ridge vent.
Plus engineer for 6" of insulation.

That keeps the heated sunny roof from radiating down into the attic space (filling it like a balloon), and helps asphalt shingles stay cooler and last longer.

That may require angling the interior corners a bit.