Is Captain Americas IW uniform really nescessary for him to be good at T3? Or is he just slightly better with the uniform?
Yes. Otherwise, you'll just have a super tanky Cap with little offense.
You can do decent scores in female ABX just with his female uniform but that comes mostly just from his T3 skill and in all other content the normal skils just don't do make enough damage.
His IW Uniform offers the greatest stat boost, so yes it's a great upgrade over the older ones