First a couple of small points that matter.
Normally 1.6 gal toilets have space left in the tank to adjust the tank height 2-3 more liters then their rated at , as a fail safe for those heavy eater crappers who need the toilet maxed to the max for a chance at a single flush.
FYI water pressure is effected by height only , not volume , meaning a 1 ft tall 1000 gallon tank has the same water pressure as a 1 ft tall 10 gallon tank.
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Costs incurred : $5 for the can of foam insulation and some styrofoam for the inner walls.
Onward:
I have insulated my toilet tank ( & the lid) to keep the cold in & reduce the tank area by 50% measured by cu. inches.
With the reduced area it allows me to raise the water level to the top of the overflow tube which is maximum flush pressure but only using 4 liters of water, instead of the normally required 8 or 9 liters.
It crippled the # 1 flush on my dual flush to using just 3 L of water, which it turns out is to little to get a proper flush out.
The #2 flush became a supercharged #1 flush.
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Doing this allows me to get a sharp flush out of the toilet using about 39% less water then its full flush rated for ,in the process reducing the thermal transfer of the tank.