On the PCT in 2004, I used a 20 degree WM Alpinlite to Tahoe, switched out for a cheap Kelty Lightyear 40 to Ashland, and got my Alpinlite back for the rest of the way.
This worked fine for me - the bag was warm enough for all those cold nights early on, and rainy/snowy nights in OR (unseasonable and unexpected) and WA (expected). The Kelty in the hot dusty days of NorCal in summer was all I needed, and I didn't bring much extra clothing - didn't mind getting the bag dirty with DEET and trail grime. If you get a WM bag, I'd suggest a full zip one, so you can open it up quilt-style in warmer weather (the Alpinlite is a WARM bag).
I wouldn't bother carrying insulated top and bottoms to make a sleeping bag stretch from 40 to 25 or whatever. Too much bulk and fuss for me. When I get to camp and I'm cold, I get in my sleeping bag and cook from there, in the vestibule of the tarptent if it's raining. It's too hot in insulation to hike with a pack on (most of the time), so I don't carry them. Perhaps if I did the PCT again I would just carry a Micropuff vest and Houdini windshirt for protection (and a MLD poncho tarp for rain/pack cover) - at least until the mosquitoes get bad! Then I would want an enclosed shelter for those few weeks. I did carry thin long-johns for sleeping when I had my Alpinlite - both for bag cleanliness and also to have a dry pair of clothes to change into after a day of hiking in the rain - I kept my sleeping clothes in the dry bag with my sleeping bag.
Part of it depends of how cold you get, etc. As long as my torso is warm, I'm fine. I hiked in shorts in the rain and snow, and was fine. YMMV. Of course, I learned a lot on the PCT and afterwards, so if I did the PCT again, it would be with a much lighter setup in general (I was more of a lightweight backpacker then, not an ~SUL like I try now.)
Cheers to your hike... it's a great adventure!