No Pin Assignments

If you mean compiling a subblock alone to determine resource usage, that's fine. The Logic Utilization number will be good and the memory and DSP block numbers might be too as long as significantly different resource balancing doesn't happen when compiling a block alone. Ignore LAB and ALM utilizations for any design that does not use nearly all the logic resources; the Fitter spreads logic out in more LABs and ALMs than necessary when it can to avoid tight packing that could impair performance or routability.

If you have report files or can recompile the original complete design, you can get the resource usage of the lower-level blocks from the Analysis & Synthesis and Fitter "Resource Utilization by Entity" tables.