I think it's better to use established tags over inventing a new one whenever possible. 

Two different address components indicating a place smaller than a barangay are very commonly found in addresses here. Examples are subdivison + purok, sitio + purok etc.

For the specific example in this changeset a possible approach would be to move "Alabang" to addr:barangay (just as we're using addr:city for all municipalities/cities independently from their place tag in OSM).

Then addr:quarter can be used for "Filinvest City". This way the order of the address components is clear for data consumers (neighbourhood before quarter). A newly invented tag like addr:estate most likely would just be ignored by data consumers and even if they consider it, they'd have no idea in which part of the address it belongs.