Lesnar is much more quick and agile than most 280 pound guys and still getting better. It's not unreasonable to think that Lesnor would beat Fedor. Fedor for the most part is a full out attacker. I believe even if Anderson Silva got up to 240 he would just counter the hell out of Fedor and win that.
To be fair I think in the Fedor/Lesnar fight whoever got it on the button first would lose. However if it went to the ground Lesnar has it in the bag, which is most likely what Lesnar would try to do.
Lesnar's only chance to win would be to smother Fedor and GnP him in much the same way he did to Mir. Fedor can beat you in so many ways. Lesnar has only a slim hope of beating Fedor. Fedor has been in with much bigger men than Lesnar and defeated them fairly easily.