No I never did weigh one, they are heavy. If you are forced to add nose weight with the .20 I would definitely install the .40 instead of the .20. Especially if the OS.40 is a .40LA. It is small and relatively light for a .40 and it will put out a little more power and more torque than a BB.25 size engine. And it's about half the price! Not too much to worry about when, sorry, if it crashes. I had a .32sx w/ a pipe on my last one and both were ruined in the crash, that hurt. If the OS .40 you have is a .40FX/AX I think it would be too much for the plane / pilot combination.
I don't remember what they give you for throws in the instructions but you may want to use those for the high-rate and about 60 percent of that for the low-rate