There are many, many different ways to secure a TD, and I don't think it's unreasonable to assume that AS would be sacrificing some of his speed and nimbleness fighting at HW.
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.