I have the Specops stock on my 500.
I would recommend it.
saves many a bruised shoulder from certain discoloration.
as for the forearm. is it a one piece plastic type deal?
or a two piece, with a nut on the front?
if it has the nut, you can change it.
but be aware that there are two different sizes.
if you buy an after market forearm.
make sure it is the right length.
and, heat-shields definitely help when your at the range,
shot after shot fairly quickly, the barrel gets HOT.
I personally had the same problem with the pistol-grip stock.
they say it's a pretty grey area.
you cant own a pistol under 21, but you can own any sort of long rifle or shotgun.
but i guess with the pistol-grip, you can use it with one hand.
I dont know.
doesnt make too much sense to me.
i went home and got my specops so they could put it on and sell it to me..
and gave me the pistolgrip when i left.
I dont get it.
Sorry for the point-oriented post.
but im pretty sure i hit all of your concerns quite accurately.

.. just for good measure.