I'd say life is very near to as good as it gets in terms of moving around chemical energy and using it to transform materials without something like a furnace or a foundry. You're never going to eat rock, it's already in a pretty damn low energy state that you cannot use for energy. Lithotrophic bacteria take advantage of redox differences between materials in rocks and live REALLY slowly so that new materials can leech in. You need to apply external energy to it in order to transform it. And as TheOtherDave has said, major alterations have happened but according to rather non-grey-goo patterns, and I suspect that the sorts of large-scale (as opposed to a side-branch that some energy takes) reactions will be more similar to biological transformations than to other possibilities.

I do think that life is not necessarily as good as it gets in terms of production of interesting bulk materials or photosynthesis though because in both these cases we can take advantage of non-self-replicating (on its own) infrastructure to help things along. Imagine a tank in which electrodes coming from photovoltaics (hopefully made of something better than the current heavy-metal doped silicon that could easily be recycled or degraded or something when they inevitably photodegrade) directly drive the redox reactions that fix CO2 from the air into organic molecules, followed by the chemistry required to take that feedstock and make it into an interesting material (along with an inevitable waste product or six). Dropper in the appropriate nutrient/vitamin-analogues and let it run, then purify it... I sometimes wonder if such a system might in the long run cause an 'ecological' disruption by being more efficient at creating materials from simple feedstocks than regular living plants and over very long timescales crowding them out, but then there is the issue of the non-self-replicating components which add a drag. Its a very interesting and potentially strange set of scenarios to be sure, but yeah not exactly grey goo (grey sprawl?).

EDIT: Percival Zhang's research at Virginia Tech may provide a look at some of the ideas I find particularly interesting:

Cell-free biofuel production:

http://pubs.acs.org/doi/abs/10.1021/cs200218f

Proposals for synthetic photosynthesis:

http://pubs.acs.org/doi/abs/10.1021/bk-2012-1097.ch015

http://precedings.nature.com/documents/4167/version/1

General overview:

http://www.vt.edu/spotlight/innovation/2012-02-27-fuels/zhang.html