Hi - your question stated 'impact on MID Servers' but I wonder if this intention was really... impact on the target machines being scanned during Discovery?? If so, the short answer for this is that it is negligible (barely noticeable, if at all). Discovery is just running commands that are native to the box which are relatively quick & painless, so perceiving any real impact to performance on the target boxes during a scan would be exceedingly rare.
Since discovery probes are different based on the target machine and the criteria you've setup discovery to use, there is no 1 answer for this general question. If this is something you are interested in measuring, you could set up a performance monitor (windows) to capture basic stat's before, during and after a discovery pass of a machine. This would provide a starting point to baseline impact.
Hope this helps?
