I like these two or a combination thereof
-Create a program in Marketo that progresses Campaign Status (Delivered, Opened, Clicked) in SFDC based on taking those actions and being a member of that specific SFDC campaign
-SDRs create a custom lead view, send using Sales Insight and use Campaign Status to monitor their leads' intereactions with the emails they've sent
So are you saying your SDRs are selecting leads and running various MSI based emails to those leads? Any particular reason they are doing this and you aren't? If it is high volume, then it sounds like marketing should do most of this and use the Lead Owner token to send on behalf.
Either way you have to manage and track it with Programs and Campaign Statuses.