I recently made a modification that got me 27 hp in third at 2800RPM, which someone told me is x 2.45 for first gear, which is 66 hp. I got a 50 ft lb in torque at 2800RPM also increase in third, so in first that would be 122 ft lbs.
Your engine will always have the same 27hp increase. The effective torque to the rear tires can be different via gearing, but since the gearing is steeper, it will travel through the power band quicker, making you arrive at a higher gear sooner. Eventually you'll have to shift, nullifying the gear ratio multiplier.
As for flywheel to rear wheel horsepower, 20% seems close from what I've seen. Maybe 25-30% if you have a big converter (4,000rpm+ stall).