I think partial transfiguration gains power by being a literal carving device. Normal transfiguration can force an object to take a shape, but they always revert. Partial transfiguration can carve pieces off selectively and have it be permanent. Eg, "the sculpture was always in the block of marble, I just removed what was not the sculpture." Although you'd need to set up incredible safety protocols. Presumably you'd transform the waste into a non-evaporative liquid while keeping a bubble headed charm on until you finite'd everything.

Harry Potter is his own little subtractive universal CNC machine. He's infinite axis; he can work on any material; he can work on any size. A mail order service could be a multi-million dollar a year business, depending on how tight he could control his tolerances. This goes doubly so because it's in 1991 compared to modern day.

Edit: Actually, I suppose sufficiently powerful wizards could do this too. They would just transfigure the whole block of steel into an engine+oil, drain it all, then finite it back so just an engine remained. And I don't think there's a big enough market for Harry to work exclusively on sculptures in the sides of the mountains or anything. Drat, foiled.