any thoughts on this, as I'm still no further
Sorry for not responding sooner, as I've been out of the office until today.
1) You will not be able to reduce the number of records inserted since you are applying the ruleset to a new column, and have no ability to modify the column where the real URL/Site name is stored. So you are adding a detail item (which might help make reports better reflect you needs), but won't actually reduce the db records.
2) In regards to performance, it's really hard to say. Since the user defined columns are only for detail data, you could actually have slower reports if the data you require could have been pulled from summary data (which is about 10x smaller and better indexed). It really depends on what you need, and how exctaly you are doing it.
3) I would expect very little log parsing performance impact unless the regex is really bad because generally the log parsing bottlenecks on the DB side and the regex would be done by Web Reporter.
4) For the user defined column config, make sure you are not using "req_line" in the free input text box. Any of the recognized columns need to be selected from the dropdown or the rule won't work. If possible, attache screenshots of the config for your rule-set and user define column tab on the log source config.