Direct answer to your question is to hook up your own time curve to the current time attribute of the cache node. I've done this alot. There is a limit to how much you can alter time. As for the quality, that depends on what you're working with.

I would say the simplest solution, the one that gives you the most control, is also quite expensive.

Evaluate your simulation at very close intervals (like, in super-slow mo), and then retime the resultine mesh animation in your 3D package... a bit like rendering in 60-120fps your 3D stuff to have a great deal of freedom to retime it in comp or editing really.

In my experience retiming of particles really doesn't work that well (especially simmed at normal rate). What you have to do is create a vector between two frames of the same particles (must be based on a unique id) then figure out how many substeps are needed and create those along the vector. After that you can retime the sim. This has obvious problems: particles dying and being born will cause errors, because there are no two frames to compare. Also the interpolation will always be linear. That being said I don't know how to achieve that in rf. In Houdini it would be quite easy though...

cheers

g