By "transfer" here I mean a thing like a curl on a large file.

It would even apply to a single socket send() of say, a 4k buffer, which is pretty common.

Broken MTU negotiation could easily result in twice as many packets as needed in that scenario.

Twice as many packets per reduced MTU hop. The worst case is pretty bad and far north of 2x the number of packets sent from origin.

The worst case is that the routers support N, N-1, N-2, N-3, which means one additional packet per hap.

I know what you are thinking: "well what if it's N/2, N/4, N/8 etc". And the answer is that in such a network you are screwed anyway. Lots of small packets would be unavoidable.