I would not recommend having your butt augmented with silicone, medical grade or not. Silicone, while it may produce good aesthetic results in the short term, will tend to migrate in the long term, leading to the potential for significant and severe soft-tissue deformation. In my practice, this is very commonly seen in people who had their faces and lips injected with silicone a decade or more ago. Apart from causing deformations, surgically removing the silicone from surrounding tissue can be exceedingly difficult due to its translucency.
A better alternative for buttock augmentation would be autogenous fat transfer, or having your own fat harvested and transferred to the buttock region. Although you may require more than one such procedure to obtain optimal results, the tissue is by definition your own, and the results and much more predicable and stable in the long run.
Best of luck.
Glenn Vallecillos, MD, FACS
Diplomate, American Board of Plastic Surgery
Beverly Hills, California