Since I bought my 5D3 last year, the yen has fallen 25% against the US dollar. That means that Canon effectively has an additional 25% headroom available to make changes to a model before they have to increase the price...
No. With a multinational company like Canon, the impact of the exchange rate cannot be so simply explained. They have many expenses that are in U.S. dollars, as well of dozens of other currencies. To take just one small example: all of their U.S. expenses (salaries, shipping costs, discounts to dealers, advertising expenses, etc. etc.) must be paid in U.S. dollars, which means they become more expensive when the Yen falls in value.
The U.S. market is important, but not even the largest market for Canon products any more. An ever-growing percentage of their sales comes from China, while European sales also represent a substantial portion of their market as well. Just as in the U.S., they have substantial expenses in these markets that must be paid in the local currency.
It is a gross oversimplification to assume that changes in the exchange rate have so simple of an impact on expenses.