On my 40g carpet tank which is 24x24x12 I run 175w with Iwasaki 14k. It's a good combo, great colour, slams out the PAR second to none and no heat issues as it's a fairly small wattage all things considered.
That said ...
Despite how much I love the Iwasaki lamp, it is a HUGE pet peeve how insanely difficult this lamp is to acquire. It is almost always a special order item and it's one of the most expensive lamps out there to boot. I'm sure any electrical savings are lost against the purchase price of the lamp. They do last longer than your average halide but that's small consolation when you're bugging vendor after vendor to see who's willing to sell you one.
... Ok I just had to check, I just checked a bunch of online vendors and the only one who lists it at all is premiumaquatics.com in the States. A good price at $70..
http://premiumaquatics.com/store/mer...vc?Screen=SRCH
... not sure what shipping or duties will be like (I assume it's subject to duties on import since it's not made in North America and thus NAFTA exempt .. but I'm not sure).
I had hoped that after 4-5 years since this product was introduced to market that it would have caught on by now and with increased demand comes increased supply (or in additional wattages like say 250w or 400w) but no such luck. In the end 175w is obscure enough that it's a victim of it's own obscurity - because 250w is not much more in wattage and there are much more choices available, in the end nobody buys 175w and so people don't realize what a gem of a light this is and thus there's no incentive for the manufacturers/distributors to increase the product line or availability. Bit of a vicious circle.