No, it's not unreasonable, but that would be in first place a problem if his opponents at HW were as fast and nimble as his opponents at MW, but they aren't. They are much slower already. So even if Silva becomes slower, the gap in speed, nimbleness and timing between him and his opponents remains the same or would even be bigger against the heavier HWs.
The only question is, how he would put on that weight to fight at HW.
Would it be a straw-man to submit that there are a number of reasons AS doesn't want to fight Jon Jones, and a big one of those reasons is likely the size and strength differential?
AS just isn't a HW. Would he be a dynamic/effective LHW? I think we can all agree that the answer is yes.
But I don't think his body at HW would allow him to do the things that make him successful at MW, and I don't think he would be able to get himself out from under any of the good wrestlers/grapplers at HW.
Unlike Sonnen, the top dogs at HW pack a helluva wallop- one or two would be all it would take.
It's a fun coversation, though.