I have a cabinetshop and I find myself on jobsites periodically that ought to be photographed but are just not ready yet for a full blown campaign. There are interesting shots to be had but the furniture hasn't been moved in, etc.

A lot of these jobs could use some improvement on lighting but there is not a lot of time for set up. Dragging a tripod along, however, is not excessive.

What are the downsides to just relying on longer shutter times or increased ISO?

If I take an underexposed shot and pump up the exposure in Aperture is there any radical degradation to the image that wouldn't be there if the shot had been taken more crisply with better lighting?

Similarly, what is the difference if I use too much light but very fast shutter speeds?

I did some low-light tripod tests at home on aperture setting.
They were shot with & without a fresnel light.
The fresnel light had a blue gel in front of it to color correct for daylight coming through the window.

In both cases the overall brightness of the shot was similar but the colors were more vivid when the fresnel light was used. The increase in vivid was what you would see if you bumped up the saturation a bit in post processing.

What are the downsides to just relying on longer shutter times?

For what it is worth, I am shooting with a Canon5D but anticipate purchasing the new 5DMk2 (for the live view option).

Most of my shots will be viewed on the internet but some of them will eventually go into a blurb book on kitchens that I want to give to prospective customers.