The problem is that if everyone implements a very large initial congestion window that just causes more network load on networks that can't handle that amount of data all at once.

So it's not "cheating" in terms of your competitors, but it's "cheating" in the sense that either you're basically jumping the queue for your TCP flows or everyone else does the same thing and then things are back to square one, if networks can't deal with it.

An argument that networks now can deal with much larger congestion windows can be made, and Google is making it in the IETF, of course.