ok let me explain the use case in details. Say I have a Sales List and AE wants to know how many contacts inside this list are already customers or prospects. Or the opposite, I want to try a new vendor and I need to know how many leads in the sample they sent are brand new. For the 2 examples, I don't want to add leads in my database. I just want to do a comparison between 2 data sources. Currently what I'm doing is, cleansing List so that it matches Marketo loading system and change format to csv. I then load List (excluding net new). I end up with a sub segment of my initial list, which I need to export, and then transform (because Marketo csv contains "," and French xls only supports ";" ). Finally I add leads to my existing List and check duplicates in .xls so that I can say these are existing leads or these are net new leads.
So I confirm that it's possible to do comparison with Marketo but I find it way easier to do it outside. The only constraint that we have (compare to other sources/applications) is that Marketo api is quite slow. So as a result, comparing a static list with Marketo full DB is taking ages. But let's say, I know for a fact that all leads in my List are based in France, I could narrow comparison to French segment or French workspace... does that make sense?