Well, yes it is. The orbiter alone masses over 99 metric tons empty. Of that, about 10 tons are taken up by the three expensive, "refurbishable" main engines. It is worth noting that only 20% of the on-orbit mass is payload.
It will be at the Saturn V level with the new extended solid boosters.
Please direct us to a URL explaining the changes in mass/thrust/burn time/flight profile/etc. with these "extended SRBs". The poor ISP and dead weight numbers for SRBs should allow only very modest improvements in total on-orbit mass due to SRB enhancements.
I also do not have much faith in NASA's follow-through on projects such as these - they have a habit of cancelling them after spending billions and not flying any new designs, with the exception of the Super Lightweight External Tank, which enabled NASA to get an acceptable payload to the ISS, but may have contributed to the Columbia failure.
It wouldn't take much to come up with a BDB version.
Shuttle is pretty much the WORST place to start for a BDB design. The plain empty external tank costs well over $60 million, the SSMEs and the SRBs also cost in excess of $50M. Low cost BDB designs cannot use much STS hardware: it is all too expensive.
Now, if you didn't really mean BDB and just meant "heavy lift booster" , then I would agree that STS hardware, in particular the SLW-ET + SSME cluster makes a fine upper stage for a heavy-lift cargo vehicle, as long as you are more concerned about performance than cost.
Just takeoff thrust and weight. Efficiency would still be cnsiderably short of the Saturn V.