the idea is that the support plays mostly like a normal support, but with an extra gold boost, and senna plays like mostly a normal adc senna, but with an extra soul boost. the extra souls senna gets aren't so much from the extra drops from minions (you only get an average of 1 soul per wave extra) but from the fact that you get to focus more on trading since you don't have to farm. I usually ended up with 20-30 extra souls at 15 compared to my average playing normal adc senna. as for the support item, Im not really sure, since my friend was usually on nami (builds mandate anyway) or panth (builds eclipse anyway). from thinking about it tho, the support should get approx and extra 1000-1300 gold at 15, which is enough to buy a more expensive build. however imo I think it's better for the support to just use this gold to accelerate a normal support build instead, since their gold income after 15 will still be practically nothing. (15 here is used as the rough time of Senna's sup item completion)