Unless it has a shiny foil or plastic facer on it, the foam is not a vapor barrier, but rather a vapor retarder. Type-II EPS would have a vapor permeance of about 1.2-1.5 @ 2" thickness, which is more vapor retardent than latex paint, but 3x as vapor permeable as an asphalted kraft facer. It's fine to go to the ceiling with it. Compressing an R19 into the 4-4.5" space between the foam & poly vapor retarder delivers an effective R of about R15-R16 at that density (compressing it down to 3.5" delivers R13- it's essentially a "fluffed" R13 batt).
If you did it that way you'd have R8.4 of foam to R15.5 of fiber (averaged) for R24, plus the R11 in the studwall, for R35 inside the vapor barrier, R19 on the exterior for about R54 total center-cavity, with a 65% ratio. You could also go with R13s in the new studwall for R56, and a 66% ratio. Either of which would be fine if you're actually in the blue. The most important thing would be air-sealing the foam. If you wanted to stop the R13s in the studwall to stay at R24 foam + compressed batt for a huge dew point margin (a 56% ratio) that would be fine- hardly noticable in the actual energy use. Put an air-barrier at the top of the cut batt in the studwall to limit convective draw.
You can't reliably fill 7.5" of space with two R11 batts- there isn't sufficient guaranteed loft. It can fill 7" well though.
If you did it that way you'd have R8.4 of foam to R15.5 of fiber (averaged) for R24, plus the R11 in the studwall, for R35 inside the vapor barrier, R19 on the exterior for about R54 total center-cavity, with a 65% ratio. You could also go with R13s in the new studwall for R56, and a 66% ratio. Either of which would be fine if you're actually in the blue. The most important thing would be air-sealing the foam. If you wanted to stop the R13s in the studwall to stay at R24 foam + compressed batt for a huge dew point margin (a 56% ratio) that would be fine- hardly noticable in the actual energy use. Put an air-barrier at the top of the cut batt in the studwall to limit convective draw.
You can't reliably fill 7.5" of space with two R11 batts- there isn't sufficient guaranteed loft. It can fill 7" well though.