Adding Boron Oxide or any other chemical will affect the expansion coefficient one way or the other. This means that you no longer have Pyrex of any particular type but something else.
As far as annealing is concerned, it is the thickness of the section that determines the annealing time. Thicker sections take a longer time because the stress across and down the section take a longer time to be relieved. This means that you need to make the mirror as thin as possible in order to relieve the stresses as fast as possible. Several different methods have been done to produce thin sections for lightweight mirrors. AXE`s method is to make tubes collapse in an airtight mold and thus make the hexagonal pattern seen with thier blanks. There is another individual that is making blanks from welding ribs between two surfaces and this works also. Then there is the Angel`s method of just casting the stuff, in his particular method he spins the mold so that there is a sag in the middle so not so much material needs to be removed.
I haven`t played around with the