What do I risk by putting a 40W tip on a 50W iron?
What you are asking makes no sense.  We have several Weller WES51 around here, so I am quite familiar with them.  The tips don't have a power rating.  The difference between the tips is the shape.  For example, the ETP tip is good for most general work, and is listed as being ".8mm conical".  The ETT tip is ".61mm conical" and comes to a slightly finer point.

These tips are just metal.  The heater and temperature feedback are in the iron.  All the tips have the same mechanical interface on the iron side.  There is no such thing as a "40 W" tip.

If the tip you have fits mechanically into the PES51 iron, then I'd say give it a try.  As long as the fit is good, the iron should regulate the temperature well enough.  Also, wattage of a temperature controlled iron only tells you the maximum it is capable of, which should be rather more than you actually use on average so that the temperature controller has some extra to work with.