So I've heard that if you go over about 350% MF you start getting more blue drops instead of rares or uniques for some reason. Is this true? If not, should I just stack as much as I can while keeping a good clear speed?

As much as you can while having high clear speed

You get more blues instead of rares because a lot of those rares are set items that dont have a set form, so they instead drop as a blue with double durability.

The first thing the game try is to give you an unique item, the more mf the more chances you have, if that fails, then it tries with set, then rare, then blue, then normal.

So, you will never get less uniques with more mf, you can get less rares because you are getting way to many uniques, sets and failed set.

(of course if you are slower, you will get less uniques overtime than with less mf and killing more, we are taking about getting less unique with the same amount of kills)

Read the guide in my sig. all you need to know