Bill, the OL Silver has virtually identical dimensions to the Rega arms, so I am sure it will fit right in. I have never actually tried it, but I can't see that it would be any different, as far as fitting under the dustcover. Also it will fit right into the Rega armhole, so it should be a totally "drop-in" swap. You might have to slightly enlarge the hole in the plinth with a half-round file, to accomodate the VTA adjusting collar.
I do have the Heavyweight on my OL Silver, and yes it is an improvement. A pretty good one. I was surprised, actually, at how much improvement I got, on an arm that was already very very good. I would definitely recommend getting the Heavyweight on the OL Silver arm. You must specify the OL end-stub model when ordering.