What is the maximum acceptable soldering temperature for a MOSFET?
If you look in the datasheet, you'll find a timed temperature profile (or a link to where that information is available on the manufacturer's web site) for machine soldering. 

The temperatures will go above 175C, and for many seconds. The effects of this one time thermal abuse have been built into the specifications for the device, it has been designed to be machine soldered. 

Unfortunately, hand soldering is much less controllable than a machine. However, several things are on your side. You can used leaded solder, which melts at a lower temperature than lead-free (the machine profile is certainly for lead-free). You can observe the tab, and stop heating moments after the solder flows. 

As there's a lot of mass to heat, use the biggest soldering iron tip you can find. Use a controllable iron, and set the temperature to enough to melt the solder, but &lt;= 260C. Practice on some dead devices. If you haven't any dead ones, practice on some live ones, and you'll soon get some dead ones. If you can do the job keeping the device within the time/temperature profile of the data sheet, then you're in with a good chance.

Where the device has an accessible tab, like a TO220, it's fairly easy. Where it's a power SO8, you can't heat the tab directly. With an etched board, the heat has to be applied from below, and the device reflowed on. When I don't have a board, I often 'dead-bug' the device, and solder heavy wires to the tab for the heatsink.