I look at it like this-
The belt system drives the fan, the water pump, the alternator, and the air compressor.
The alternator is not placed under a high demand anyhow, and it's demand (engine load) will NOT change with engine RPM. No gain.
The water pump is very small relatively speaking, (as are most all in a full flow cooling system design) and even when the thermostat is closed, the full flow of coolant is required to redistribute heat within the engine from a few crazy hot areas to the entire rest of the engine. I wouldn't want that underdriven.
The air compressor, like the alternator is a fixed demand, but it's a little different. While it's pumping you could free up a couple of ponies (literally, a couple) by pumping slower, but then your air does not build as quickly as it should. While the compressor is more than adequate, I wouldn't call it overkill, and surely not something I'd want to compromise on since I don't believe it's something that you'll feel from the driver's seat.
It really comes down to the fan. There may be a mathematical formula (I'm sure there is...) that could approximate the actual horsepower it costs at any given RPM, but I don't know it. Based on other published facts and some extrapolation (read-my best estimate), I think you're looking at fifteen plus, maybe eighteen ponies at the rated speed. That's something right there, but underdriving that alone (if it could be done...) is going to net you three or five of that back, and still compromise the cooling on the few times that you need it.
I'd look into the electric fans. They still place their full load on the engine (via the alternator) but only when they're needed, and realistically I don't think the actual "on-time" will be all that much. Finding one large enough however may prove a challenge. A member here,
SasquatchSanta played with this some on a bobber if you want to search through his posts. He's a little north of you and he still felt the need to put his original fan back in place during the summer months. He had an LDS installed, but it was turned back so far that I don't think it was making the horsepower (and thereby the heat) that an LDT in original trim would make.