I might do more in-depth videos if I see this build becoming popular (and riot blind to it).

With this build, in theory at 15 min, you will have a worth of 4100g (gold + stats), as opposed to a perfect farming adc who will have 4250g.
That is if you play bad (only complete supp item at 15min, no extra souls on enemy champion, no pressure on lane, no plates).

This is not as good, but pretty close, and your aggressivity should make the ennemy adc miss some farm on top.
With this strat, Senna and the Support must pressure the lane as much as possible (support should be aware of ganks etc.) and ensure senna can stack souls easily.
Senna must take 2 melee/wave to avoid gold reduction, and maximise stats (cannon included).
Honestly, your support item should be complete at around 10min and you should be the first one to get your mythic every game (even if someone has 2 kills).

The pros:
you will be richer than anybody, should be stronger than any other typical ADC, and completely own teamfights (both front to back and flank).
Your laning phase should be very strong, and should ensure the enemy ADC never gets to 3 items.
Having twice the number of wards is a delight, trust me. Your team will have a better macro.
The best thing: you can give up farm whenever its a bit hard to get it. Honestly this will probably be the reason why riot nerfs it.

the cons:
A lot of people will troll you when you buy a support item and will want to farm your waves. This is typically 80% of my losses.
You will need to stick with your support until the end; but that will also force the ennemy support to stay with its ADC if you manage your wave decently enough.
I would recommend having a duo with you who knows how to play the strat.

I estimate with a decent duo, you should get a minimum of 60% at your elo (if not more).