what effect if any will d springs have on a 92fs? how does it affect reliability?
will they last as long as the standard springs? does anything else have to be changed to use them?

It lightens your DA pull slightly and makes the trigger feel better overall.

It has no effect on reliability, after all, that's what Beretta uses in the "D" to begin with.  Springs lighter than the "D", may have problems with hard primered ammo.  

It's a Beretta part, built for a Beretta 92, so it will last as long as any 92 part.

You don't have to change anything else.  Just take out the FS spring and drop in the D spring.  Done.

Both the full frame and compact use the same spring.  The strut is shorter on the compact, but that is the only difference.

It's the single best mod you can make to an FS.  It gives you a better trigger without any decrease in reliability, costs about $5 and takes about 5 minutes.  

Do it!

The D spring is exactly the same as your existing one, the only difference is it's a little shorter.  If your up to it just cut off 1/4"

I used a cutting wheel and my dremmel.  When your done cutting use the flat surface of the cutting wheel to flatten out the bottom of the spring like it was before you cut it.  It's not necessary but it gives it that finished look.