rharris389 :
With that motherboard though? I've been told it's not a great board to fit stuff around. (Sorry if one of those is that board, I just can't tell)
Those people were probably concerned about the VRM heatsinks which are pretty tall and would be under where the H100i mounts if you place it on the top of the case. Looking at the left side panel of the 300R, there are two mounts that allow for side by side 120mm or 140mm fans. You might be able to mount the radiator on this panel but you might have to install the fans on the outside of the case. If you look at the red and black rig, it actually has what appears to be an Asus Maximus motherboard with very similar VRM sinks right under the radiator. There should be clearance but if you are worried about it enough, you could look at a different case, a different motherboard with smaller VRM sinks or a AIO with a 120/140mm radiator to mount on the side panel. The NZXT Kraken X40/X41 and Corsair H90 both offer similar performance to an H100i due to their 140mm rads and at low fan speeds are probably close to even with it due to the advantage that 140mm fans have - they can move as much or more air than a 120mm fan at a lower RPM. The best thing to do is to search for pics of that board in a 300R to see if the VRM sinks overlap the top fan mounts.