^^^ If he ends up like Johnson last year I think that's great, and means it really is a good time to nab Spiller.
Johnson started last year even worse than Spiller (first three games: 11 carries for 4 yards; 8 carries for 17 yards; 14 carries for 24 yards and no TDs in any of them). After those first three performances, if we project Johnson's stats from there across a full season he rushed for (the equivalent of) 1,477 yards, and 7.5 TDs.
That's without even considering receiving yards, which I think Spiller definitely has the edge on Johnson for.
Of course, on the other side of the ledger though Spiller has to deal with Fred Jackson whereas Johnson didn't have anyone else capable behind him, so there's obviously a bit more risk there.
(FWIW Johnson and Cam are the two guys last year, IMO, who started so slow that a lot of fantasy folks didn't notice that they put together pretty nice seasons by the end).