300 mm/s (I find it easier to think in terms of /s since that is what almost everybody uses) is nearing the absolute limits of printing speed. I like to print conservatively at no more than 80 mm/s. It is a sliding scale of print times vs. quality. For boxes and brackets and things, bang them out as fast as it can go, but for figures, print slower for better quality.
You may want to check out some of the published FFF profiles, there are a few links on the "M2 tips" thread in the tips forum. I believe jimc's are in there somewhere, and a lot of people have used rsilvers' profiles linked on Thingiverse as a good place to start.
I would recommend you get comfortable with PLA before trying too much ABS. It is easier to work with and you will have fewer adhesion and warping issues. One advantage of ABS over PLA for figures is that you can use acetone vapor to smooth it (see
my recent thread), and overhangs seem to print without as much curling for me, but that's about it. If you want a smooth model, try a few coats of high build automotive primer and sand it back down to the plastic.
For a large build, you could also consider printing in sections and joining the sections afterwards. That can reduce the need for support by printing different parts in different orientations, and since you are interested in making a mold the different thread orientations won't matter as much to you if you are going to post process them.
