In general, I don't see much of a difference. When you open files affected by the plugins, however, you're relying on plugin code, so things can change.

For example, I have an XML file that is 15 MB. Before I installed these plugins, Quicklook would take about a second to show it as a simple text file, basically. Now it takes much longer (about 10 seconds) but the output is nicely formatted (well, "nicely": it seems to fail pretty quickly on one CDATA node, so most of the document is not formatted correctly... but I digress). Clearly QLColorCode is adding a big overhead when invoked.