I haven't used DSP Builder, and probably will not for some time as we build industrial cameras over here. But I have used resource sharing before, but it was just stuffing a pipelined cordic.
Saving resources always comes at the expense of more work, unfortunately but then it shows off great when we get the job done with the limited resources anyhow ...
Coming back to the original start of this: can you humour me and explain, in the upsampling case of a 100 MHz system / DAC clock and a 45 MHz sampling rate, where the 45 MHz sampling rate comes from?