C/AS2 requires a property rating of 120 minutes for CA risk group firecells. Is it reasonable to use the Burnout formula from C/VM2 to use a lower figure, when the remainder of the fire safety design uses C/AS2? I know it is a little naughty, but is that opinion founded on logic?
I don’t think your suggestion is naughty at all. It’s called ‘an alternative fire design’.
It 's not uncommon to have a C/AS2 design with a ‘deviation’ for a specific area of the design.
The deviation should be clearly documented to enable a third-party to review. You will need to makes sure you identify all the C2-C6 performance requirements that could be affected by this deviation and addressed in the design.
A word of caution though, if you based the fire severity on a specific activity type, it may limit the use of this space/building to be used in future for other purpose that may have a higher FLED. I note that Risk Group CA activities are included in 400MJ/m2 and 800MJ/m2 activities in Table 2.2 of C/VM2.