There is quite a bit more in the SOPs than just the flying the aircraft part, and some of that might be counterproductive, simply daft or way too long winded for the intended operation. And of course yes, one can use other than the OEM procedures, but should be very sure that his legal department agrees with it.
That said, when i tried to change an SOP i wrote emails to the relevant guys and at usually some ASRs as well, especially if those SOPs were not particularly safe (single point of performance calculation for example which me and a few others changed with ASRs in one company). Underpinning your point with helpful examples, pointers to industry standards and being polite in ones language does help though, but that is just normal stuff anyway.
Quite a bit of the current airbus SOP set seems to be squarely aimed at the lowest common denominator in training possible. And that is needed in certain companies, but not in all. SOPs have to go in line with the prevalent company culture, and have to be relevant for the operation and the type of personnel used, otherwise parts of it will not be followed which creates a negative confirmation bias for the whole set.