unfortunately you got the wrong info.  I've seen a few posts here that said the Studio XPS 435 MT P041K heatsink will fit in the Studio XPS 9100. I was able to find and buy one of those. But just my luck, it seems to line up, but the screws on this unit are not long enough to attach it to the motherboard, and they don't look to be threaded the same as the screws from the P281K.  435MT motherboard has pre-tapped holes around cpu socket to mount its heatsink.  9100 has clear untapped holes and use a separate Dell back plate to mount heatsink. You can buy a Dell XPS 730X heatsink which uses same back plate as 9100, or even better get a Coolermaster Hyper evo 212 and get rid of the Dell backplate.  
Hello,  I recently had a couple of the plastic tabs on the heatsink of my Studio XPS 9100 that hold the screws in place break. I've looked everywhere for a replacement for the heatsink P281K and had no luck. I've seen a few posts here that said the Studio XPS 435 MT P041K heatsink will fit in the Studio XPS 9100. I was able to find and buy one of those. But just my luck, it seems to line up, but the screws on this unit are not long enough to attach it to the motherboard, and they don't look to be threaded the same as the screws from the P281K.  Does anyone have a solution for this? You'd think that using the screws from the P281K would work, but I don't see a way to get them off without breaking the little flat washers. Thanks, I really appreciate your help. 
Studio XPS 9100, looking for a heatsink