There is a big difference in the earnings multiplier for higher tiers. AY Plat gets you 25% additional points (only on AY), BA Gold gets you 100% additional (double points) on BA, AA and IB. And for the past few years they have a 2x or 3x promotion during the autumn. You will not get the 100% extra when flying AY though unless you book it with a BA flight number (I have booked HEL-JFK in the past with a BA flight number for this reason). For LHR-HEL all the AY flights have a BA flight number as well (I always book them with BA flight numbers for this reason).

There are upgrade "vouchers" with BA EC, they are called gold upgrades for two (GUF2 on this board) and also there is GUF1. You get them at certain tier point thresholds and they are applied at time of ticketing. One voucher is valid for a whole ticket, not one segment. So if you have LHR-SIN-SYD return that would be one GUF1 voucher to upgrade the whole trip. They are only for BA operated flights though, in the same way that AY vouchers are only for AY flights.

I would not enjoy all those long haul flights in Y though. With my current employer I pay the difference (normally about 300-400euro per trip) to be ticketed in premium economy (which AY doesn't have). From london the CX premium economy to HK is really nice and a good chance of an op up to their business cabin which has much better seats than BA or AY. For the extra money I get some comfort and also much higher avios and tier point earnings.