I don't recall the P40 hinge material (I have one but it's been a long time ago). If it is the CA type hinge, then change them--Dubro's work well.

If it is a plastic pin hinge (like a Dubro), as long as the movement is completely free, then I would probably use them. If they are sticky, and it isn't something trivial to fix them (like removing a little flashing), then I would replace them with Dubro. Remember you will have something like 10 to 14 hinges moving in every control input, so a little friction will add up fast.

Everyone replaces the Brodak CA type (looks like thick, stiff silkspan) hinges. Why?

It is like truck stops.

A survey of truckers at a Diner were asked, “Why did you stop here to eat?” The most common answer was, “There’s a lot of trucks here, and you know what that means!” 

So, ask why replace the hinges? Because a lot of guys do it, and you know what that means!

The reported “disadvantages” of the stock hinges are stiffness (particularly new) and lack of durability. How true are those statements?

Who knows? Everyone replaces the stock hinges, ya know. 

   Larry Fulwider

CA hinges are stiffer. Stiff. Work the controls on an ARF with the stock hinges and there's noticeable drag. Carefully installed pin hinges are smoother, much less drag. Free working controls allow the flier to maneuver a plane when there's little or no line tension. Could be the difference between a splat and a recovery. Also, free working controls are more accurate when doing the tricks.

Bingo! 

got some pin hinges. going with them. thanks