If you're riding 4.8mph up an 8% hill for 0.8 miles then, as others have said, it doesn't matter what gear you are in you will use the same amount of power.
Using a smaller gear will reduce the amount of force (force or torque is different from power) you need to apply with each pedal stroke but you will have to increase your cadence to maintain the same speed and use the same amount of power over the climb. If you reduce the force and keep the same cadence then you will just go slower.
For example:
If we take your 4.8mph up the 8% hill and you are in the 30x25 gear then you are pedaling at 50.5rpm. If you put a cassette with a biggest sprocket of 28 teeth on the back then you could climb that hill at the same speed and overall power at 56.5rpm. Each stroke will require about 13% less force (so that will feel easier) but you need to do 13% more strokes every minute (which will feel harder).
As 12strings said, while in theory your power numbers stay the same there becomes a point at which pedaling requires so much force that it becomes difficult (if not impossible) for a human to output the necessary force. For climbing a cadence of 60-80 rpm is good although lately even higher cadences (90rpm) have become popular.
Personally, I'd say mashing up the hill at 50.5rpm is not an ideal situation and that you may find it more comfortable to put a bigger cassette on the back. Even 56.5rpm could still be considered to be mashing as for climbing you should probably be aiming for a cadence of 60-80rpm. In your 30x25 gear doing 60-80rpm would mean a speed of 5.7 to 7.6mph. In a 30x28 gear that would be a speed of 5.1 to 6.8mph.
There are only two ways to achieve those sorts of speeds. Either increase your power output or reduce the weight you are hauling up hill. And reducing the weight would probably be the easiest thing to do. If you reduce your overall weight by 5% (9.5lbs) then when it comes to climbing that weight loss will almost completely translate into a 5% speed improvement (especially up an 8% hill).