By default 50% of the FAST Cache capacity is spread equally among all LUNs ensuring that every LUN gets some space in SSD for it's busiest data. 

The other half of the FAST Cache SSD space is reserved for LUNs to use on an as need basis. 

I think of it much like a private and a public space.  The private space is a little piece that is carved out for every individual LUN to use, but the public space is used by the busiest 64K chunks in the busiest LUNs as needed based on which chunks are the "hottest".

any new granularity, can i disable Fast Cache for individual pool LUN ?  Any additional improvements in terms of detecting Fast Cache "un-friendly" workload (sequential) and not promoting it ?

Roger that...if it hits SSD in a FAST VP pool, no need to move it to the SSD in FAST Cache.  That SSD cache space is reserved for the "stuff" that still lives on spinning media.

same rules apply where if block is already in FAST-VP tier (flash), it will not be promoted ?