I don't think you would be 'wasting' funds on a large booster. With current infrastructure and size of the boosters, it would cost about the same for multiple EELV launches as the Launch of an ET with two RS-68 core booster, even with development. I have convinced myself that an in-line payload gives the most upward capacity enhancement. From the basic core plus 2 SRB's you could then add various sized liquid boosters: 2 H2-A first stages, 2 RS- 68 first stages, 2 double RS-68's, and finaly replace the SRB's with RS 68's. This gets you to 150 ton throw boosters with little development cost. Also heights well with VAB constraints. If anyone is really serious about lunar development and Mars, this is the way to go.