Obviously, but economies of scale are about costs, not capacity. A plant with double the capacity costs less than twice, especially if development costs (of both the product and the machinery) are taken into account. The part of the spread of mass production in the PV price cuts in recent years is generally recognised.

Money poured into wind turbine production will help fuel increases in the size of wind turbines

As well as their price, and also the distance between two neighbouring turbines in a wind farm. It's not that easy to evaluate whether unit costs reduce. (You also confused the economies of scale from the increase of manufacturing capacity and the economies of scale from the increase of the output of a single plant.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Me neither as I haven't seen a link to a full English translation, but you could have read my short summary of rates upthread.

what is the rate after the three years?

First, the Japanese FiT seems to be like the German one, that is, producers get a fixed rate set at the time of grid connection and guaranteed for 20 years (with sub-10-kW, that is residential rooftop PV being the exception, getting it for 10 years only). Second, this rate for new installations can be revised by future governments. The current law wants to maintain the current level for three years, and as far as I know the degression thereafter is not set.

As Germany and Spain have shown, two to three years of mania can produced a PV solar sector dominated by rooftop installations

Yawn. We have been through this before: no, the Spanish and German solar booms were quite different, with on-ground solar dominating in Spain (due to a lack of differentiation of rates; Japan's 10/20-year running time differentiation will further enhance the effect). What has been similar in both countries however is that legislators wanted to curtail on-ground installations (because of conflict with agriculture), not rooftop.

consume the majority of FiT budgets while producing a small fraction of the renewable power in a country

We have been through this, too.

• There is no such thing as a "FiT budget", that is a bucket of money different modes would need to compete for: the (open-ended) costs are spread to utilities and via them to customers.
  
• The difference between market price and feed-in tariff is not constant, nor power plant portfolio independent due to marginal pricing, and due to the correlation between the daily peaks of PV production and demand, there is a significant effect.
  
• It is not a goal of feed-in laws to somehow balance the income of different renewables. The main goal is to create a sufficiently large market for the development of each. You could make a point about the relative profitability of investing in different renewables, but the total amount of FiT reinbursements is no measure of that.
  
• Last year electricity generated by PV in Germany (19.0 TWh was already in the same order of magnitude as hydro and wind (19.5 resp. 46.5 TWh).

The development of domestic industry (with cheap foreign competitors as a given) doesn't depend on the feed-in law alone, but tariffs, industrial subsidies and quality control, too (low-quality solar cells don't produce as much for as long, a reason Chinese producers haven't killed European ones off completely). But my point was not restricted to new manufacturing (what you speak about) or even just manufacturing.

the 2015-2020 period is likely to be one in which natural gas prices in Japan plummet as LNG imports from the US resume

You said his before, but didn't explain what it effects will be (hence my question "by the "FiT regime" that may be harmed, do you mean the institutional framework or the plants working under the FiT taken together, or the whole renewables sector (including suppliers)?").

they simply aren't efficient enough to warrant their integration into the grid

What meaning of "efficient" is this now? I think a contribution like below is rather efficient, however:

as a matter of decreasing CO2 emissions, and creating fuel price declines, support for solar water heaters is probably a much better choice.

You make it an either-or... Germany has both. (And the neolib anti-solar lobbyists in the federal government attacked both, not just PV.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}

What amounts are we speaking about? I tried to look, and found these figures:

• There is one US LNG terminal project connected to Japan with a planned capacity of 8 million tons a year,
  
• one billion cubic feet of natural gas generates 0.1 TWh,
  
• 1 million tonnes of LNG is 48.7 billion cubic feet,
  
• if I calculated right, these LNG imports would give Japan 40 TWh of electricity annually.