I think you should go ahead an email them. It's not too late -- I got a responses from two PIs who want to set up a meetings to chat about my work before I apply yesterday. They're busy, we're busy. It's a little late, but don't worry about it too much. Don't get me wrong, it's super intimidating and I basically had a panic attack for each letter I sent ? But here are the benefits:
They get a chance to talk with you and connect with you before you apply. It puts a face and personality to your name/application, and, according to my mentor, makes them more likely to champion your application/fight for you to get in. I like this part because they get to see how excited I am, lol, and they seem to like that.
You learn more about the current direction their research is going. The papers they have published are snapshots of their research an unknown number of years/months ago. They could still be interested in the same things, they could not. You don't know. But if you find out, you have the chance to incorporate that into your personal statement.
Apparently some mentors will not admit students they haven't talked to. It's hard to tell from the outside who these individuals are.
If they know your current mentor, it's a chance to network.
In some places, it's an expected part of the process and not doing it is a faux pas. (Unless their program/web page says not to email, I would do it).
You get a sense of them as a person. Would they be good to work with? Are they kind? Are they supportive? How do they talk about their current/previous students? Getting in is one thing, being in an environment that supports finishing is another.
You get a sense of their mentoring style. There are a lot of great people in my field who I might share research interests with, but we're incompatible insofar as mentoring style goes. My ideal would be someone who will meet with me regularly (weekly, biweekly) but who is still pretty hands-off. I don't do well with someone who wants to micro-manage me and the direction my research takes (think the junior colleague model of mentoring).
Don't get me wrong, there are a number of places in the application process where you can find these things out. But when you consider the benefits, I think the downsides don't mean much.
Here's a thread on GradCafe from last year that helped me out a lot. I recommend taking a look: