In Go, is there any aji left in this corner enclosure (3-5, 4-4, 6-3), and if so, how can it be invaded?
With the addition of the P17 stone, the enclosure is considered safe. The standard response to white R17 is black R16. R17 is a probe rather than an attempt to live, and in practice black's best answer will depend on the surrounding stones.

See these two links for more information:

http://senseis.xmp.net/?443563Enclosure

http://senseis.xmp.net/?4435Enclosure

Even without the presence of the P17 stone, the best white can get locally is a ko.

If you search this pattern on eidogo.com, you'll see that this enclosure usually ends up as black territory.