I'm pretty sure that you do get passive buffs as a Super Saiyan. I mean, why wouldn't you? They probably add to your base stats, since Super Saiyan in itself is more or less a buff, though a glorified blonde haired one.
it inreases crt rate,and not sure does it increase crt power.And yeah it uses more HP and ep if i am right.
basicly my personal oppinion is,higher lvl of kaioken is good for namekian warriors since they have ton of HP to spare,so they can sustain it without problems and can play on low hp.As for other classes i dont see the point of increasing the level of kaioken because you can stack the effect as pizzapie explained.
Also you could max it out if you make some nuker class nuker build of spiritualist and you can have High dps output.
does kaioken increase your ability to sustain it in higher levels or does having level 3 just increase critical hit rate and not attack and such because if it just increases your hit rate and drains more energy from what i know i should stick with 1 but does 3 allow you to reach higher kaioken levels like x10