I think one of the US SW20 models came with a 70A alt. The lightweight alt bracket racerx makes comes with a washer/shim so you can use the smaller alt.

WIDEMR
Yeah biggest saving will be the bracket for sure. 
 
I will be running a smaller alternator then the 100 yes.
 
From factory, can the 80 and 100 be swapped? or do they have different thickness mounting?
 
Ian are you able to measure the thickness of the bottom mounting tab/bolt location? The 100A i have is 58mm.
 
Suppose smaller output will be same or smaller if anything, can always shim it out with some washers.
 
BTW the same manufacturer/design/technology alternator ect on the same car, the 100A wont have any more load on the motor then the 80A
 
Another trick is to turn off the alternator (cut the ign wire to it) to stop the load on the motor when required (during short runs or TPS over 95% ect) and gain a few KW's, but thats a different topic.
 
Ill be keeping the alt in the factory location, no vac or dry sump pump, dry sump would be nice but alot of weight, more connections and parts mean more things to fail ect, ill test the car, if i dont need it i dont need it.
Only spot to relocate is AC location i guess, but much hotter and higher COG.

Given the same loading the alternator will not load the motor any more.  If you are going to turn the alternator 'off' by disconnecting the excitation current you'll certainly want to make sure sure all your trims are tuned correctly.   Fuel pumps and injectors do not operate the same at the reduced voltage, even if it is only a few volts.

Also a smaller output alternator will have smaller and, generally, lighter rotor, meaning less reciprocating mass, light weight fly wheel lovers will be all over this, so will spool up faster.

 
Under load tho i dunno how its affected.
 
 

Racer X Aluminium bracket (bottom, top and adjustment rod) 846 grams

http://www.lynxae.co.uk/Products-alternators.htm

Dunno if you have seen these guys before, but their 140a alternator weighs in at 6kg, and they make their stuff with denso parts for most things.

As for "switching off" an alternator, i would only attempt it on a 3 wire alternator with a l-ion battery, as the voltage drop would be catastrophic otherwise.

Depending on ecu choice, you could switch to a 4 wire off a later model car to duty cycle control the alternator, so it doesn't load up when you don't need it to, and i'm assuming this is what Brendan is referring to, as thats how a lot of modern cars are. For example with a pnp m800 for sti's and evos the computer will switch off the aircon compressor and run the alternator at minimum duty to maintain voltage at high throttle inputs.

Ultimately your 100a requirement will be the killer, you can find some 3kg 80a 4 wire alternators fairly easily. Maybe you should adapt a suzuki swift or similar steering coloumn into your car and run a manual rack.....

Might be time to go 80A and see what happens....

Yeah there is a reason manufacturers do thing like that. Just wish they did a hollow cast alloy one instead.

Nothing lockwire cant fix