Lower tRFC will scale up performance, with diminishing returns, as far as you care to take it because less time is spent refreshing. However, stability will get flaky very rapidly as you near the minimum values a particular set of ICs can tolerate at a given clock speed. The difference between completely stable (or as close as one can get) and having to reinstall your OS can be razor thin.
tREFI works similarly to tRFC (former is time between refreshes, later is how long a refresh lasts), though higher is better and gains tend to be more linear. The line between stable and unstable is usually much more subtle though as well as more temperature dependent.