Your answer doesn't make sense. The Amazon and others protected sites in Brazil are a much larger area than 8%. If we preserved only 8%, Brazil would be the richest country in the world. Maybe the US devastated an Amazon to get there.
Destroying the world's most biodiverse ecosystem (seriously, read some statistics on number of species in the Amazon -- it's mind boggling) to grow the GDP by 10% is not just incredibly selfish (supposedly exchanging a global treasure for national gain). It's not even the best for our own interests; it's myopic, greedy, inhumane. Even the actual direct economic value of scientific research with industrial applications such as biomimicry research, plants with applications in medicine and food industry, etc. would completely dwarf a one time 10% gain. And then there's the hardly measurable value of lost knowledge and natural richness.
Don't be the child exclaiming "But my brother was greedy too!". This is our treasure, and our responsibility.
If greed ends up winning and most of the forest is destroyed, Brazil will find itself without a national treasure, and likely be economically worse off for it.
Some of the worlds major economic success stories are countries with few natural resources, Taiwan being a common example. At the same time, there are a number of countries heavily exploiting their resources, yet seemingly without alleviating their poverty.
There are plenty of places trying to re-forest, which also suggests that you might get stuck with the long term cost of trying to undo the damage done.
In Michigan we have a few little pockets of real old growth forest and national forests the size of a small country.