The manufacturer recommend a higher watt because there are a lot of bad psus out there. 550 is overkill for a r9 380.
r9 380 = 198W at the socket, meaning thats 158.4W from the power supply itself, if it is a 80% efficient (any bronze really).
an r9 380 system (all components), is hardly even pulling 300W total at the wall, so the psu would just be pulling 240W. therefore 550W is beyond overkill, even for overclocking.
look for the correct (or enough) amount of amps on the +12V rail, and make sure the power supply quality is high enough.
edit: looking at the video, we have to assume thats measured at the socket, so
371 * .8 = 296.8
from the power supply. so technically, if the quality is high enough, a 300W PSU would suffice. i dont know of any, but highest quality would then be a 400-430W.