The leads are a bit overstresssed by that much current, as compared with the ratings for wires., but I assume the silicon is going to be dissipating considerably more heat than the wires, and it's mounted on a heat-sink. Plus the leads are good up to the melting point of solder, so their exact temperature isnt of that much concern.
If I were using them though, I'd keep the leads as short as possible and transition to more capable thickness of copper wire asap.
No. Look at the IRF1404 device. It says it is rated at 202 amps. However, note 6 says that the TO220 package itself is only rated for 75 amps. Fairchild doesn't play this game. For instance, their ISL9N302AS3ST has even lower Rds but they rate the device at only 75 amps.
So, it appears that about 75 amps is the TO220 package limitation.
I note that the nominal lead cross-section for that part is 0.050" wide by 0.020" thick, or 0.001 square inches. I compare that with the "fusing current" (the current that can melt a wire in a nominal room temperature environment) for copper wires: 16AWG at 0.051" diameter,
0.002 square inches, is rated at 117 amps. This tells me you'd better get the heat out of those leads by conduction to something cool nearby if you really want to run 280 amps through them! They CANNOT be very long, or you won't get the heat out of them without immersing them in something that will take away the heat (e.g. fast-moving air or some coolant fluid).
Another way to look at it: the 0.001 square inch cross section will give you about 0.67 milliohms per inch at room temperature (and higher at higer temp: over 0.8 milliohms per inch at 70C). At 280 amps, that's 52 watts per inch of lead, at room temp; over 60 watts per inch at 70C.
Maybe you can operate it in a liquid CO2 bath. ;-)
Maybe you mean liquid N2 - CO2 can't exist as a liquid at atmospheric pressure. But, you could put some dry ice in some kind of freon - in grade school science, we would put dry ice in acetone to freeze stuff. :-) FWIW, the grocery store where I shop has a deep freeze with dry ice for sale.
You could look at some of those "overclocking" web sites to get some ideas for some really aggressive cooling.