For just shape corrections, I´ve had great result with the opal ingots. If you are going to,for example, lengthen the incisors, they can be to transluscent. So in my experience the opal ingots(specially the Opal1) needs tooth structure underneath. In some cases that increased trans. works in your favour, all dependent on the case. The opal ingots have a great chameleon effect.
If it´s a light shade, I have also had success with value ingots. Darker shades HT ingots(if you can keep the material thickness quite thin and uniform). My experience with thin veneers/no prep veneers is to stay away from LT ingots, unless you want the material to mask discoloration.
As to thickness, no problem pressing 0,2-0,3 shells. Just be sure that the shells are about the same thickness all over. If the restoration goes from thick at the sprue - to thin in the middle third - to thick at the cervical, you can come across difficulties getting a full press at the margins sometimes.
