We had teak decks on our Hylas before building a 575 in 2017.    After spending spring and fall cleaning and sealing the decks, spending money on tung oil, etc. and fixing sikaflex seams,  we decided to try the Flexiteak on the new boat.

Its now been 7 seasons and we are happy with it.    It does not go grey, it largely maintains the original shade chosen.    It cleans up easy with simple stuff like cleanser and a ruffie pad.   You can even pressure wash it.    Like teak, it can get hot in the sun.    You can buy replacement sections on boatoon.   Repairs seem simple, but the one I needed (right after I took delivery,  an unrelated Hanse problem required cutting open a section and fixing it) required someone skilled.  It looks fine and is still holding up OK.   Oh, and its anti-skid properties are better unless you let the grain rise on your grey teak.

I would not mix teak and flexiteak.   To me its an either/or proposition.    If one section of real teak goes bad,  I'd probably just repair it with the same material.