Just sharing my modified Captain, I thought I could avoid the modifications since it may be camera angle issues but it really doesn't look so right out of the box. And I'm quite unsatisfied with the Winter Soldier series so far, seems like almost every figure has some sort of issues.
Out of the box
The figure looks really flat and has not much definition on the chest area. It's my opinion though.
Modifying
I struggled to take out the undershirt even with the tutorial, it's really tight and the main reason that's causing the arms to flare up like a chimpanzee. And my undershirt has splotches of superglue all over the place, wtf.
Using epoxy putty, (I used Tamiya, a whole pack of it in fact.) I layer the chest and shoulder slowly. I have to lightly sand the area first to get some grip (real important since the putty will peel off if you don't do that) It's a slow process and and you have limited working time with the putty (a few hours at most), I spent like about 4 hours shaping the chest, which I feel is the most important area. Remember to leave the putty in the water so it doesn't sticks to you like superglue. Makes it easier to work with also.
Occasionally I would check the progress by comparing images and making sure it still looks buff instead of man boobs.
Side view, looks more buff instead of just flat.
Finished
Besides the sculpting, I also took the time to make some adjustments to the undershirt, cut 2 lines on the front and leave the back as it is to allow some flexibility. Now I can do this without looking forced.
All in all, the modifications do make the figure looks more like the prototype. BUT I'm really unhappy with how Hot Toys are basically forcing us to do something to their figures which they should have done right in the first place.
Hope this helps anyone looking to do some modifications.