Moving torque curve to the left?
 

I understand that we have to (install smaller diameter exhaust pipes than oem, warm air intake, smaller flowing injectors if possible).

Not aware of anyone actually doing it but I'd like to try it. I don't think it'll work any miracles all by itself; probably cam and intake changes needed too.

Are you trying to increase low end torque? Advancing the cam(s) is an old trick. What are you working on?

advancing the cam, long runner intake, long tube headers with the right size piping, longer stroke, camshaft with less duration and lift.

What kind of car are you working on?

+1 on advancing the cam or even a different cam grind.

And making the exhaust smaller won't do what you think. It will move the torque peak to the left, yes, but due to greater backpressure at high rpm - it won't actually INCREASE torque at low rpm.

Then have the ECM reprogrammed.

a smaller diameter pipe will add low end power., headers for street driving use small diamiter primary tubes and a longer collector, my mpg's did go up when i installed them, and the race headers use bigger primary tubes and shorter collectors and hurt low rpm but add highend, like frank said in and of itself it might not work out that well, as your engine from intake runner length/diameter,to cam lift/overlap/duration/advance,to compression ratio,valve size,and exaust all need to be matched (with-in reason), exaust pulses at certain rpm's are a certain size and velocity ,and when they all follow each other out without "bunching up" (to big of a pipe), or running into each other (too small of a pipe) it is more efficent. ,engine designers have to design the engine to operate over a broad range of rpm, if we never go above 2000-3000 rpm ,i'll bet we would see cars with much smaller pipes, ,,,,,But i could be completly wrong about all of this! :D

The reason you use a smaller diameter header primary is to strengthen the exhaust pulse to scavenge the cylinders. It only works in a certain rpm band. The typical headers you buy at Summit and Jeg's are tuned for 5000-6000 rpm - they don't pulse scavenge at 2000 rpm. You'd have to make the header primaries 8 or 10 feet long to make them work correctly. the best you can hope for at hypermiling rpm's is to reduce backpressure as much as possible. Theoretically that would mean using the biggest headers possible, but any header on the market is going to be plenty big for hypermiling.

For the rest of the exhaust system, I seem to recall John Moss (GM engineer responsible for the 94-96 Impala SS) telling us that the stock system was sized correctly for best economy at 55 mph - minimum diameter that won't cause a restriction. The only thing you need to change in the stock exhaust is the muffler - stock mufflers are fairly restrictive.

Tri-Y headers are the ones you want.