You might study the smallnetbuilder review, which tears down the innards. It looks like there had been a small internal fan at one point. Generally, it looks like the idea is for ambient air to enter the bottom, wrap around the board and over the top big heat sink and out the center top.
I was having heat problems with an 87U and got into it a bit. It has a similar design, and my anecdotal observation is that these flat routers are less efficient than the vertical designs like the 56U or 86U. But the 87U was particularly bad, because it didn't have enough openings in the top. It seemed like the idea was for hot air to leak out the sides. I took off the top and running it that way helped a bit but not as much as I expected. I put a pair of infinity fans on either side so that air was blown/sucked left to right across the board, and that helped a lot. I took off the heat sink and the shields and there were some problems with the thermal pads being misaligned, but not enough to make much of a difference. I ended up putting a small fan in the cover over the CPU and wiring it to one of the pinouts. That dramatically lowered the temperature, almost as much as the fans blowing across.
Unless you are going to install a fan, I wouldn't bother opening it up, and I wouldn't bother taking off the heat sinks or worrying with the thermal pads. I would start with a little Infinity USB fan on the bottom blowing up, which might help with the underside heat sinks but isn't going to help the top heat sinks. Then I would move it to the top, sucking out, and see if that helps. I expect that to be the best. They also have pairs of fans that you could hamburger the router with.