Stackexchange has a good discussion on the topic:
https://bitcoin.stackexchange.com/questions/1863/why-was-the...
[1] http://www.tik.ee.ethz.ch/file/49318d3f56c1d525aabf7fda78b23...
If you think 40% is not good enough, okay, let's target 48%. That's still just ~6.5 rounds per block (~82 seconds per block). Bitcoin's parameter of ~47 rounds per block is clearly overkill; the authors' graphs don't even go that far out (see figure 3).
Interestingly, the first paper you linked mentions that although Ethereum was intended to use single-level GHOST scoring, uncle blocks aren't actually counted toward chain difficulty, so it really has the same security model as Bitcoin. This was also confirmed by an Ethereum dev [1].